Before Rome — the early calendars
The month names we use today are Latin, and the Latin names are themselves the product of two earlier traditions: the Greek calendar systems of the Aegean city-states and the Etruscan calendar of central Italy. Neither tradition gives us a month name we still use, but both shaped the structure that the Romans inherited. The most striking inheritance is the count: in both traditions, as in the original Roman calendar, the year had only ten months.
The Greeks never had a single shared calendar. Each city-state ran its own, aligned to its own civic year and named for its own local festivals, with the result that Athens, Sparta, Corinth, and Thebes ran on overlapping but non-identical month-grids. The Athenian Attic calendar had twelve lunar months named for festivals — Hekatombaion (Hekatombaia festival), Metageitnion, Boedromion, Pyanepsion, Maimakterion, Poseideon (sacred to Poseidon), Gamelion (the marriage month), Anthesterion (the flower festival), Elaphebolion (the deer-hunt festival), Mounichion, Thargelion, and Skirophorion. The Athenian year began at the summer solstice, not in spring, which made coordination with other Greek states a constant administrative headache. The pattern of naming months for festivals rather than gods is a Greek innovation the Romans only partially adopted.
The Etruscan calendar is more directly ancestral to the Roman one. The Etruscans ran a ten-month year that began around the spring equinox, with the first month roughly corresponding to the Roman March. We know less about Etruscan month names than Greek ones because the Etruscan language is only partially deciphered and very few calendrical inscriptions survive, but the structural inheritance is clear: ten months, spring start, no fixed names for the two missing winter periods. The Etruscans seem to have treated the dead of winter as a kind of administrative void, not because they did not know it existed but because they did not bother to regulate it with a calendar. This explicit omission becomes one of the most charming features of the early Roman year.
The original Roman calendar, attributed by tradition to the founder Romulus and therefore dated to the eighth century BCE, was a ten-month system that began on 1 March and ran through the end of December. The months were named, in order, Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December. Of these, Martius through Junius are named for deities, and Quintilis through December are simply numbered: fifth, sixth, seventh, eighth, ninth, tenth. The year had roughly 304 days, leaving a winter gap of about 61 days that was not assigned to any month at all. Romans of the early Republic literally did not have names for the days between December and March; the period was simply 'winter', and no civic or religious business of any consequence was conducted during it. The calendar reform history guide picks up the structural story from this point and follows it through to Gregory.
Numa's reform — around 715 BCE
The credit for filling in the winter gap goes, according to the standard tradition, to Numa Pompilius, the second king of Rome and the legendary founder of much of Roman religious practice. Plutarch's Life of Numa, written in the late first century CE, gives the most extended ancient account of the reform, and Macrobius's Saturnalia, written around 400 CE, gives the most detailed technical reconstruction. The two sources do not agree on every detail but they agree on the broad shape: Numa added two new months at the end of the year — Januarius and Februarius — to convert the ten-month system into something more like a real lunar year of twelve months.
The reformed Numan year had twelve months totalling either 354 or 355 days, depending on the source you trust. Most months had 29 days, except for March, May, July (then still Quintilis), and October, which had 31. February had 28. Plutarch is explicit that the asymmetry was driven by Roman superstition about even numbers: Numa deliberately gave each month an odd number of days because the Romans of the period considered odd numbers more auspicious than even. February, the new short month at the end of the year, was the unavoidable exception, accepting an even count because the year had to balance somewhere.
The Numan calendar still drifted against the solar year because 354 or 355 days is about ten or eleven days short of the actual 365.24, and the system relied on a periodic intercalary month called Mercedonius to keep the months roughly aligned with the seasons. The intercalations were a political decision made by the Pontifex Maximus, the chief priest, and by the late Republic that office had been thoroughly weaponized by factional politics, with intercalations being inserted or omitted depending on which consul or magistrate wanted his term extended or shortened. By the 40s BCE the system was a wreck, and Julius Caesar's reform in 46 BCE swept the entire intercalation problem away by switching to a purely solar 365-day year with a single intercalary day every fourth year — the rule we still essentially use, refined slightly by Gregory in 1582. The Julian-to-Gregorian converter walks any date across the 1582 cut.
January — Janus
Janus is the Roman god of doorways, beginnings, transitions, and gates. He is almost always represented with two faces, one looking forward and one looking back, which makes him the obvious choice for the month that sits on the threshold of the year. The name Januarius is a straightforward adjectival derivation from Janus, meaning roughly 'belonging to Janus' or 'sacred to Janus', and the OED traces the English form January through Anglo-Norman Genever and Middle English Januarie back to the same Latin root.
What makes Janus the right god for the month is less about his place in the pantheon (he is a relatively minor deity overall) than his function. Janus is explicitly the god of liminal moments — the start of journeys, the opening of gates, the entry into new buildings, the morning, the new year. His shrine in the Roman Forum, the Janus Geminus, was a small temple with double doors that were kept open in time of war and closed in time of peace. The doors were rarely closed; Roman historians made a point of recording the few occasions they were, because peace was historically anomalous. Plutarch and Ovid both make Janus's role in opening the year an extension of his general function as the divine opener.
The question of when January became the first month is more complicated than the etymology suggests. The original Numan calendar placed January as the eleventh month, immediately after December and before February, with the year still beginning in March. The civil year shifted to a January start around 153 BCE because the Roman Senate moved the inauguration of consuls from 1 March to 1 January — the new consul-elect for the year needed time to muster troops and travel to his assigned province before the spring campaigning season, and bringing the inauguration forward by two months was an administrative convenience that had nothing to do with the calendar itself. The civil year then quietly followed the consular year, and Julius Caesar's 46 BCE reform codified 1 January as the start of the calendar. Whether 'Janus opens the year' is a religious justification that came first or a rationalization that came after the consular shift is a question Macrobius (Saturnalia I.13) addresses but does not definitively resolve. The most likely answer is that Janus's symbolism made him an obvious patron for the new start once the start was already there, not that his cult drove the administrative decision.
The Roman custom of giving small gifts on 1 January — a tradition recorded by Ovid in the Fasti — is the direct ancestor of the European New Year's gift practice that survives in some traditions today. Romans exchanged dates, figs, honey, and small coins as Janus-gifts to ensure a sweet start to the year, and the practice spread across the empire and outlasted the empire itself.
February — Februa
February is named not for a god but for a festival. The Februa was a Roman purification ritual held in the middle of the month, on or around the 15th, in which priests beat people (or, more precisely, the women who wished to conceive) with strips of goat-hide called februa as a fertility and cleansing rite. The word februum itself appears to be a pre-Latin Italic term meaning purification or expiation, possibly borrowed from Etruscan, and the month Februarius means simply 'the month of the Februa'. The OED traces English February through Old French Feverer and Middle English Feverer and February, with the modern spelling owing more to Latin scholarly influence than to the natural English sound change.
The Februa festival is the same Roman feast usually called Lupercalia, conducted at the Lupercal cave on the Palatine hill where Romulus and Remus were said to have been suckled by the wolf. The Luperci, the priests of the festival, ran through the streets of Rome naked except for goat-skin loincloths, striking bystanders with their februa strips. Plutarch in the Life of Caesar records that Mark Antony, who was a Luperci, used the festival of 44 BCE — held just a month before Caesar's assassination — to publicly offer Caesar a kingly diadem, an offer Caesar refused on the day but which Antony's gesture was clearly designed to test. February's name is therefore tangled up with the run-up to the assassination that gave us the name Julius for the month five.
The 28-day length is the more famous oddity. February has 28 days for a chain of reasons that begin with Numa's preference for odd-numbered months: somebody had to be the even month for the totals to balance, and the obvious candidate was the last month of the year, which in the original Numan ordering was February. When Caesar's reform converted the Roman year to 365 days and a 366-day leap year, the 28-day February was preserved with a 29-day leap variant inserted at the very end. The traditional Roman leap day was the bissextus, a doubled sixth day before the Kalends of March (24 February), inserted into the calendar literally as a repeated day. The English term 'leap year' is unrelated to the Roman naming; it derives from the Anglo-Saxon observation that fixed annual dates 'leap' forward by two weekday positions in a leap year rather than the usual one. The modern arrangement, where the leap day is inserted as 29 February, is a medieval rationalization of the Roman bissextus.
The 29 versus 28 rule produces about a quarter day per year of error against the solar cycle, which is what the Julian rule of one leap year in four was designed to correct. The Julian rule still slightly overcorrects, by about eleven minutes per year, which is why Gregory in 1582 added the rule that century years are not leap years unless they are also divisible by 400. The next time this matters noticeably is 2100, when the Gregorian rule will drop a leap day that the Julian rule would have kept. The multi-calendar converter handles every leap-year rule across every system.
March, April, May, June — Mars, Aphrodite, Maia, Juno
The four months from March through June form a coherent block: all of them are named for deities, all of them retain their names from the original Romulan ten-month calendar, and all of them sit in the productive half of the Mediterranean year between the spring equinox and the summer solstice. Together with Janus, the deities of these months were the Roman gods most directly associated with the rhythms of agricultural and civic life.
March (Martius) is named for Mars, the Roman god of war and the father of Romulus and Remus. Mars is more than the war god in Roman religion — he is also a god of agriculture, of boundary marking, and of the protection of the fields, with his archaic country variant Mars Silvanus and his civic variant Mars Ultor representing two sides of the same divine function. The choice of Mars for the first month of the year is overdetermined: he is the father of the founder, the patron of the campaigning season that began in spring (Roman armies marched in March), and the god most associated with the spring rituals of land consecration that began the agricultural year. The OED gives the etymology as straightforward derivation from Mars through Old French Mars to English.
April (Aprilis) is the contested one. The most popular ancient derivation, recorded by Macrobius and Ovid, is from Aphrodite (Latin Venus), the Greek goddess of love. The argument runs that April was the month sacred to Venus in the Roman religious calendar, with the festival of Venus Verticordia on 1 April and the major Veneralia later in the month, and the Greek-derived name acknowledges the goddess as the patron of the season of flowering and procreation. The alternative, recorded by Varro in De Lingua Latina, derives Aprilis from the Latin verb aperire, meaning 'to open', referring to the opening of buds and flowers in spring. The OED gives both etymologies as possibilities and declines to choose. Modern philologists tend slightly to favour the aperire derivation because the Aphrodite derivation requires explaining the loss of the initial sound and other irregular changes, but the question is genuinely open and both classical sources were guessing.
May (Maius) is named for Maia, an obscure Roman goddess associated with growth and fertility, identified in later syncretic religion with the Greek Maia (the eldest of the Pleiades and the mother of Hermes). The Roman Maia was originally a separate figure, sometimes called Maia Maiestas, associated with the earth-mother and the growth of crops, and her major festival fell on 1 May with an offering of a pregnant sow to her shrine. The OED gives the derivation as Maius from Maia, through Old French Mai to English May. The folk etymology connecting May to maiores (the elders, the ancestors), recorded by Ovid as an alternative theory, is now generally rejected.
June (Junius) is named for Juno, queen of the Roman gods, wife of Jupiter, goddess of marriage and the household, and the patron deity of women in their reproductive and matrimonial capacity. Juno's role as marriage goddess is the standard ancient explanation for June's persistent association with weddings, a connection that has survived in Western tradition for more than two millennia (June is still the most popular wedding month in much of Europe and North America, partly for climatic reasons but partly for genuinely ancient cultural ones). The OED traces June through Old French Juin back to Latin Junius. An alternative ancient theory, recorded by Ovid, derives Junius from juniores (the young), parallel to the rejected Maius = maiores derivation, and is similarly not supported by modern philology.
July — Julius Caesar
July, originally Quintilis (the fifth month of the old Romulan ten-month year), was renamed Julius by the Roman Senate in 44 BCE in honour of Julius Caesar, who had been assassinated on the Ides of March (15 March) of that same year. The choice of Quintilis was deliberate and personal: Caesar had been born on 12 July 100 BCE, by the calendar then in use, which made Quintilis his birth month. The senatorial decree of 44 BCE, recorded by Suetonius in the Life of Julius Caesar, renamed the month as a posthumous honour, and the new name was in use within months of Caesar's death.
The OED traces English July through Anglo-Norman Julie and Middle English Julii back to Latin Julius. The English pronunciation with stress on the second syllable ('joo-LIE') is a relatively recent development, attested only from the eighteenth century onward; earlier English pronunciation followed the Latin stress on the first syllable, and 'JOO-lee' is still the standard pronunciation in some British regional varieties and in many traditional poetic readings.
The naming was politically loaded in a way that is now hard to recover. By attaching Caesar's family name to the prime summer month — the month of his birth, the height of the Mediterranean season, the period of major Roman religious festivals — the Senate of 44 BCE was making a permanent claim about Caesar's place in Roman civic life. The decree came in the immediate aftermath of his assassination, in the chaotic period before the Second Triumvirate formed and before the rise of Octavian (later Augustus), and was driven largely by Mark Antony's faction. The honour was unprecedented: no living or dead Roman before Caesar had been given a month named for him, and the naming converted Caesar's personal name into a permanent feature of the civic calendar. The precedent set in 44 BCE made it inevitable that Augustus would be given the same honour, and the precedent set by Augustus made it inevitable that later emperors would try to extend the practice. Several emperors did try — there were short-lived attempts to rename other months for Caligula, Nero, Commodus, and Domitian — but none of those names survived their patron's death, and the calendar today preserves only the names of the two emperors who matter most.
August — Augustus
August, originally Sextilis (the sixth month of the old Romulan calendar), was renamed Augustus by the Senate in 8 BCE, sixteen years after the death of Julius Caesar and forty-four years before the death of Augustus himself. Unlike July, this was an honour given to a living emperor, and the choice of month was again meaningful: Sextilis was the month in which Augustus had achieved his most consequential military and political victories, including the conquest of Egypt in 30 BCE that gave him control of the Roman world. The senatorial decree of 8 BCE, recorded in Macrobius's Saturnalia I.12, justifies the renaming on the explicit grounds that Sextilis was the month of Augustus's greatest deeds.
The most persistent myth in the etymology of the months attaches to this renaming. The story, repeated in nearly every popular reference work since the nineteenth century, is that the Senate moved a day from February to August so that the month named for Augustus would have the same 31-day dignity as the month named for Julius. The story is almost certainly false. Its earliest surviving source is a thirteenth-century manuscript by the English astronomer Johannes de Sacrobosco, written more than 1,200 years after the events. The contemporary Roman sources, including the reconstructed fragments of the original Julian calendar from inscriptions of the first century BCE and CE, show that the alternating 31 and 30 day pattern with a short February was established by Caesar's astronomer Sosigenes of Alexandria in 46 BCE, well before Augustus was born. August was already 31 days. February was already 28. There was nothing for the Senate to redistribute.
Why the myth has been so persistent is itself an interesting question. Sacrobosco was a serious scholar whose works were used as university textbooks for several centuries, and his version of the etymology was widely copied without verification. The story has the satisfying narrative shape of imperial vanity reaching across centuries to shape the modern calendar, and that shape has made it irresistible to teachers and popular writers for generations. The actual story — that two emperors got their names attached to months because the Senate of their times decided to honour them, with no day-stealing involved — is less dramatic but more consistent with the evidence. The fact-checking site Snopes, the Library of Congress reference desk, and several recent classical scholars (notably the relevant entries in the Oxford Classical Dictionary and the OED) have all explicitly debunked the myth, but the popular form keeps reappearing.
The OED traces English August through Old French Aoust back to Latin Augustus, which is itself the cognomen Augustus took as a title (it means roughly 'venerable' or 'majestic') rather than his birth name (which was Gaius Octavius). The English usage of 'august' as an adjective meaning 'majestic' or 'venerable' is a direct descendant of the same title, by a separate but parallel route through Old French.
September through December — the mis-numbered Latin
The remaining four months are the strange ones. Their Latin names are simply ordinal numbers — Septem (seven), Octo (eight), Novem (nine), Decem (ten) — and in the original Romulan ten-month year these were genuinely the seventh, eighth, ninth, and tenth months. The year began in March, and counting through Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, you arrive at September as the seventh. The names were never opaque or mysterious; they were simply descriptive of position in a calendar that no longer exists.
The mismatch arose when Numa added January and February at the end of the year around 715 BCE. The civil year was now twelve months long, but the original ordinal names of the last four months were not renumbered. There are two explanations in the ancient sources. Macrobius in the Saturnalia argues that the names were preserved out of religious conservatism, because the months were associated with specific festivals and renaming would have required altering the religious calendar in ways the Roman pontiffs were unwilling to undertake. Ovid in the Fasti simply notes the mismatch as a curiosity without offering an explanation, suggesting that even in the first century BCE the issue was already a familiar piece of antiquarian trivia rather than an active problem.
The mismatch deepened when the start of the civil year shifted from March to January around 153 BCE. September was now no longer the seventh month even relative to a March-start year (which is what the name still implied); it was the ninth month of a January-start year, and the gap between the meaning of the name and its position in the calendar widened from zero to two. The Romans had two opportunities to renumber — the addition of January and February, and the shift to a January start — and they did not take either. The names have remained off by two for roughly 2,150 years now.
The OED traces each English name through Old French (Septembre, Octobre, Novembre, Decembre) back to Latin (September, October, November, December). The spelling and pronunciation have been remarkably stable across this long transmission. The only persistent variation is in the syllable count: in formal modern English, all four names are normally trisyllabic (Sep-tem-ber, etc.), but Middle English and Early Modern English often used a four-syllable variant (Sep-tem-ber-e) that preserves the final unstressed vowel of the Latin. The trisyllabic version is the modern norm.
There is one further subtlety. Some scholarly readers have argued that 'Septem-ber' cannot etymologically be the seventh month at all, because the Latin suffix -ber does not have a clear ordinal meaning. The standard philological response is that -ber is the Latin ordinal-formant -bris (as in imber from a base meaning 'rainy month', not an ordinal), which had fused with the numerical stem by the time of the earliest attested usage. The ancient Roman authorities, including Varro and Macrobius, all read the names as ordinal, and modern philology agrees. The four month names are genuinely numbered seven through ten in a calendar that has not existed for 2,700 years.
If you want to see the structural consequences of the renumbering across calendars, the Roman numerals date converter is the focused tool for the Latin counting system in date contexts, and the multi-calendar converter lets you compare the Roman names against the equivalent Hindu, Hebrew, Hijri, and Persian month systems.
What other languages call them
The Latin month names spread through Europe with the Roman Empire and survived the empire's fall. Today, every major European language except a handful in Eastern Europe retains visibly Latin month names, transmitted through different medieval routes and modified by different sound changes. The English forms (January, February, etc.) come through Old French after the Norman Conquest; the French and Spanish and Italian and Portuguese forms come more directly from spoken Vulgar Latin; the German forms come more directly from medieval scholarly Latin. The variation between Januar (German), January (English), Janvier (French), Gennaio (Italian), Enero (Spanish), and Janeiro (Portuguese) is the regular outcome of applying each language's normal sound changes to a common Latin source.
The Slavic languages mostly went a different way. Polish, Czech, Croatian, Ukrainian, and Belarusian retain pre-Christian Slavic month names that describe seasonal phenomena rather than Roman gods. The Polish month of January is Styczeń, which derives from a word meaning 'connection' or 'meeting' (referring to the year-end transition). February is Luty, meaning 'fierce' or 'cold'. March is Marzec, the one Latin survivor. April is Kwiecień, the 'flower month'. May is Maj, another Latin loan. The pattern continues, with Slavic month names competing with Latin ones throughout the language family. Russian, by contrast, switched fully to Latin month names — Январь (Yanvar), Февраль (Fevral), etc. — during the eighteenth-century Petrine reforms that modernized Russian civic culture along Western lines.
The Old English month names form a fascinating lost system that did not survive the Norman Conquest. The Venerable Bede, writing in the early eighth century, records the Anglo-Saxon month names in De Temporum Ratione, and they are nothing like the Latin ones we use today. Old English January was Æfter-Yule (the month after Yule). February was Sol-mōnaþ, the 'mud month'. March was Hrēð-mōnaþ, named for the goddess Hrēð. April was Ēaster-mōnaþ, named for the goddess Ēostre, whose name survives in modern English Easter. May was Þrimilce, the 'three-milkings month' (because cows could be milked three times a day in that month). June was Ǣr-Liþa and July was Æfter-Liþa, the 'before-Litha' and 'after-Litha' months. August was Wēod-mōnaþ, the 'weed month'. September was Hālig-mōnaþ, the 'holy month'. October was Wīnter-fylleþ, the 'winter full moon' month. November was Blōt-mōnaþ, the 'blood month' (the month of livestock slaughter for winter storage). December was Ǣr-Yule, the month before Yule.
All of these were obliterated by the Norman Conquest and the subsequent Frenchification of the English vocabulary, and by the fourteenth century the Latin-via-French names had completely displaced the Anglo-Saxon ones in everyday use. Ēostre survives only as Easter; Yule survives as a poetic term for Christmas; the rest are recoverable only from medieval texts. The contrast with German, which retained Latin month names but resisted French influence more generally, is striking: both languages converged on the Latin system but for very different reasons.
The non-European calendars solved the problem differently. The Islamic Hijri calendar has its own purely Arabic month names (Muharram, Safar, Rabi al-Awwal, etc.), the Hebrew calendar has Hebrew month names of Babylonian origin (Tishri, Cheshvan, Kislev, Tevet, etc.), and the Chinese calendar uses simply numbered months (first month, second month, etc.) with no proper names at all. None of these systems borrowed from Latin even when Latin had become the global administrative default, and all three are still in active religious or cultural use today. The Hijri converter, Hebrew calendar, and Chinese calendar cover each of the three major non-Latin month systems still in active use.
Why we still use this
The Roman month system has survived for roughly 2,700 years with only one major modification (the Julian-to-Gregorian leap rule of 1582) and no successful rationalization of the names. We are still using the same twelve labels — two for emperors, four for gods, one for a purification ritual, four for ordinal positions in a calendar that has not existed since the eighth century BCE, and one for a contested derivation between a Greek goddess and a Latin verb. The system is a museum piece. The question is why we have not replaced it.
The proximate reason is inertia. The Latin month names were so deeply embedded in European administrative and religious practice by the late medieval period that displacing them would have required coordinated action across hundreds of jurisdictions, and no jurisdiction had the authority or interest to lead the effort. The Catholic Church standardized the names through its liturgical calendar; the Holy Roman Empire used them in imperial administration; the rising Atlantic empires of Portugal, Spain, France, and England exported them globally. By the time anyone with the power to change them might have thought it worth doing, the cost of conversion vastly exceeded any imaginable benefit.
The deeper reason is that month names, unlike most other features of the calendar, are essentially arbitrary labels. The mismatch between September's meaning and September's position is a curiosity, not a bug. The names do not affect the underlying solar arithmetic; they do not interfere with leap-year calculation; they do not produce ambiguity in dates. They are just inherited Latin words that happen to identify the twelve sections of the solar year. A rationalized system that renamed September to 'ninth-month' would carry no technical advantage over the current system and would lose two millennia of literary, religious, and historical association. The cost-benefit is genuinely worse for renaming than for keeping.
The one serious historical attempt to replace the system was the French Republican calendar of 1793-1805, which renamed all twelve months with new agricultural and seasonal labels invented by the poet Fabre d'Églantine. Vendémiaire (the wine harvest), Brumaire (the fog), Frimaire (the frost), Nivôse (the snow), Pluviôse (the rain), Ventôse (the wind), Germinal (the germination), Floréal (the flowers), Prairial (the meadows), Messidor (the harvest), Thermidor (the heat), Fructidor (the fruits). The system was internally consistent and astronomically elegant, but it failed within twelve years for reasons largely unrelated to the month names themselves — Napoleon abolished it in 1805 to reconcile with the papacy and because the ten-day Republican week had failed in practice. The month names were never the problem; the working week was. The French Republican calendar converter lets you compute the Republican date for any Gregorian date, and the experiment is worth knowing about as the only nearly successful modern attempt to rename the months at all.
We will keep the Roman names. The off-by-two ordinals will keep being a curiosity that everyone notices once and then accepts. February will keep being short. Augustus and Julius will keep being remembered, in a small way, every summer. The Latin month system is now the longest-running uninterrupted administrative labelling system in human history, and there is no plausible political path to its replacement. If you want the broader history of which calendar reforms succeeded and which failed, the calendar reform history guide is the companion piece on the structural side. If you want the modern date format issue, the date formats by country reference covers which country writes the names in which order.
Further reading: Macrobius's Saturnalia (Loeb Classical Library, three volumes) is the standard ancient technical reference on the structure and history of the Roman calendar. Plutarch's Life of Numa (in the Parallel Lives) gives the most extended ancient account of the Numan reform that produced the twelve-month year. Ovid's Fasti is the major Roman poetic source for the religious associations of each month and is the closest the ancient world produced to a poetic etymology dictionary for the calendar. The Oxford English Dictionary entries for each month name give the standard modern philological accounts of the transmission from Latin through Old French to modern English, with full attestation histories.