Which day of the week were you born on? With a little practice you can work it out in your head in a few seconds, using a method invented by the mathematician John Conway. This guide teaches the doomsday rule step by step.
The idea behind the doomsday rule
In every year there are a number of dates that always fall on the same weekday, which Conway called doomsdays. In 2026 they are all on a Saturday. April 4, June 6, August 8, October 10 and December 12 are all Saturdays in 2026. If you know the doomsday of the year, you only need to count from the nearest doomsday date to your date.
The method has three parts: find the doomsday for the century, adjust for the year, and then count from a nearby memory date.
Dates that share the doomsday weekday
- Even months share their day and month number: 4/4, 6/6, 8/8, 10/10 and 12/12.
- Odd months pair up with opposite numbers: 5/9 and 9/5, 7/11 and 11/7.
- The last day of February, which is the 28th, or the 29th in a leap year.
- March 0, which means the last day of February, and January 3 (or January 4 in a leap year).
Step 1: the century anchor
Each century has an anchor weekday. For the 1900s it is Wednesday, for the 2000s Tuesday, for the 2100s Sunday and for the 1800s Friday. The pattern repeats every 400 years, so the 2400s are Tuesday again. Most of the dates you need will be in the 1900s and 2000s.
Step 2: the year
Take the last two digits of the year. Divide by 12, and note the whole number and the remainder. Then divide the remainder by 4 and ignore the rest. Add the three numbers together, and move forward that many days from the century anchor.
For 2026: the last two digits are 26. 26 divided by 12 is 2 with remainder 2. The remainder 2 divided by 4 is 0. The sum is 2 + 2 + 0 = 4. Tuesday plus four days is Saturday, so the doomsday of 2026 is a Saturday.
Step 3: count from the nearest memory date
Find the doomsday date closest to your date. Say you want the weekday of May 17, 2027. For 2027, the last two digits are 27: 27 divided by 12 is 2 with remainder 3, and 3 divided by 4 is 0, so the sum is 5, and Tuesday plus five days is Sunday. The doomsday in 2027 is therefore Sunday. The nearest memory date is 5/9, which is May 9, and a Sunday. From May 9 to May 17 is eight days, one day more than a week, so May 17, 2027 is a Monday.
A second example: a historic date
What day was Norway's constitution signed on May 17, 1814? The century anchor for the 1800s is Friday. The last two digits are 14: 14 divided by 12 is 1 with remainder 2, and 2 divided by 4 is 0. The sum is 1 + 2 + 0 = 3, and Friday plus three days is Monday. May 9 is therefore a Monday, and May 17 is eight days later, which is a Tuesday. That matches the history books.
Why the method works
A common year has 365 days, which is 52 weeks and one extra day, so each date moves forward one weekday from one year to the next. In a leap year the shift is two weekdays after February 29. The step in the method, where you divide by 12 and by 4, is a clever way of adding up these yearly shifts without counting each year. The result is a rule that you can do in your head and that never needs a table.
Leap years
In leap years the memory dates for the first two months move one day: use January 4 and February 29 instead of January 3 and February 28. Leap years are the ones divisible by 4, apart from the century years that are not divisible by 400. For the dates from March onward there is no difference.
Practice
Start with the date of today and work out its weekday, then check with a calendar. After a few tries, the memory dates become automatic. Then try with birthdays and holidays. For example, December 24 is twelve days after 12/12, and since 12 mod 7 = 5, Christmas Eve falls five weekdays after the doomsday. In 2026 that is Saturday plus five, which is Thursday.
If you want the answer immediately, the weekday tool on this site gives it for any date you type in.