How it works
Pick any date to see what day of the week it falls on, whether it is a birthday, a historical event or a future appointment. The result also shows the day of the year and which occurrence of that weekday it is in the month (for example, the 3rd Monday).
Dates use the Gregorian calendar, which most countries have used since the 1700s or earlier. Dates before a country switched from the Julian calendar may not match historical records.
Formula
Zeller's congruence: h = (q + ⌊13(m + 1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ + 5J) mod 7
Questions people ask
What day of the week was January 1, 2000?
Saturday.
Why do calendars repeat?
The Gregorian calendar repeats exactly every 400 years. Individual years repeat more often: a common year starting on the same weekday has the same layout, typically every 6 or 11 years.
How can I work out the weekday in my head?
Methods like Conway's Doomsday rule use anchor dates (4/4, 6/6, 8/8, 10/10, 12/12) that always share a weekday within a year, then count forward or back.