What day of the week was it?
Enter any date, your birthday, an anniversary, a moment in history, and instantly find out which day of the week it fell on.
How the day of the week is worked out
Days of the week repeat in regular cycles: a common year pushes the calendar forward by one weekday (365 = 52 weeks + 1), a leap year by two. Formulas like Zeller's congruence build on that pattern, the same logic your browser applies here in an instant. A handy bit of trivia: within any single year, 4/4, 6/6, 8/8, 10/10 and 12/12 always land on the same day of the week.
Why the calculator starts at 1583
In October 1582 the Julian calendar was replaced by the Gregorian calendar, the one we use today. To realign the seasons, ten days were skipped: Thursday, October 4 was followed directly by Friday, October 15. Not everyone switched at once, though: Britain and its American colonies held out until 1752, when they dropped eleven days. For earlier dates, the "historical" weekday followed the Julian calendar, so this tool accepts dates from 1583 onward, where the result matches the modern calendar.
The calendar repeats, but not every year
A normal year shifts the days by one, a leap year by two: if 1 January 2025 is a Wednesday, in 2026 it is a Thursday. It is why a birthday «moves» forward one day a year, with a jump of two after 29 February.
The full cycle closes every twenty-eight years, when weekdays and leap years line up again: the 1998 calendar is identical to the 2026 one. The cycle breaks at non-leap centuries, such as 1900, so it does not cross 2100.
Why the calculation starts in 1583
In October 1582 the Julian calendar was replaced by the Gregorian one, and ten days were deleted: 4 October was followed by the 15th. The dates between 5 and 14 October 1582 simply do not exist, so they have no weekday.
The change was not adopted everywhere at once: Protestant countries waited until the eighteenth century and Russia until 1918. That is why a historical date from the seventeenth or eighteenth century can have two readings, and serious sources always specify «old style» or «new style».
What changes with distant dates
The Gregorian rule removes three leap years every four hundred: century years are leap only if divisible by 400. The year 2000 was, 1900 and 2100 are not. It is the exception that derails mental arithmetic and more than one hastily written program.
With very distant dates it also matters that 1 January has not always begun the year: in many Italian cities the year started at Christmas or on 25 March. A date before the sixteenth century must be read knowing which style the document used.
Nearby tools
For the days between two dates there is Difference between dates, and for leap years Leap year. The week number of a date comes from Week number.