What day was it? What day will it be?

How the weekday for any date is calculated

The Gregorian calendar repeats every 400 years, so once you know the anchor day for any year you can derive every other date in that year with simple arithmetic. Mathematicians like Zeller and Conway have published one-line formulas for this; modern browsers expose the same answer through their Intl date APIs. We use the browser's calendar engine so the answer matches whatever localised conventions you have set on your device. The math is faithful all the way back to the year 1, using what historians call the proleptic Gregorian calendar (the Gregorian rules projected backwards across centuries that actually used the Julian calendar).

Why dates before 1582 get tricky

The Gregorian calendar was introduced in 1582 to fix a slow drift in the Julian calendar. Different countries adopted it at different times: Britain not until 1752, Russia not until 1918. If you are looking up the weekday for a date in, say, the Battle of Hastings, the answer this calculator gives you is on the proleptic Gregorian calendar, which is what most modern history textbooks use. For dates after the relevant country adopted the Gregorian calendar, the weekday matches what locals actually called the day. For older dates, scholars sometimes prefer the Julian-calendar weekday for cultural fidelity; we do not show that here, but it differs by a small offset that grows by one day every century.

Roman calendar weekdays and other curiosities

The Romans named the seven weekdays after the seven planets they could see in the sky: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. The Latin name appears at the top of every result on this page. If you want to keep exploring, the days between dates tool tells you the exact gap between any two dates, and the week number calculator shows the ISO or US week for the same date.