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.