📆 Weekday Calculator

How to Find the Day of the Week for Any Date

The Gregorian calendar repeats on a 400-year cycle — after exactly 400 years, every date falls on the same day of the week again. This 400-year period contains exactly 97 leap years and 303 regular years, totalling 146,097 days, which divides perfectly by 7. This means if you know your birthday's weekday today, it will fall on the same weekday again in 400 years. Within that cycle, the calendar also repeats in shorter periods: years that start on the same day and have the same leap-year pattern share an identical weekday layout.

The Doomsday Algorithm

The Doomsday algorithm, devised by mathematician John Horton Conway, lets you calculate the day of any date mentally. The trick relies on "doomsday" — a set of easy-to-remember dates that always fall on the same weekday within any given year. For example, 4/4, 6/6, 8/8, 10/10, 12/12 are all the same weekday every year. So are the last day of February, 5/9, 9/5, 7/11, and 11/7. Once you know what weekday "doomsday" falls on for the target year (computed from the century and year), you can mentally jump to any date. Conway could name any date's weekday in under two seconds.

Interesting Date Facts

Friday the 13th occurs at least once in every calendar year, and can appear up to three times. September 13th is statistically the most common Friday the 13th. January 1st falls on a Sunday more often than any other weekday over the 400-year cycle. The 13th of any month is slightly more likely to be a Friday than any other day, due to the uneven distribution of days in the Gregorian calendar. Days of historical note: July 4, 1776 (US Independence) was a Thursday; April 15, 1912 (Titanic sinking) was a Monday; July 20, 1969 (Moon landing) was a Sunday.