Doomsday Weekday Calculator
Find the day of the week for any date using the Doomsday algorithm, with the steps shown.
Day of the week
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The Doomsday rule, devised by John Conway, uses a handful of memorable "doomsday" dates that always share a weekday within a year, so you can work out any day of the week in your head.
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How to use it
- 1
Pick any date
Choose a date from the calendar — past or future.
- 2
See the weekday
The day of the week appears instantly using the Doomsday algorithm.
- 3
Follow the steps
Review the century anchor, the year's doomsday, and the nearest doomsday date.
Perfect for every need
Learn the mental trick
Practice the steps so you can work out any weekday in your head.
- Mental math
- Memory training
- Party trick
History & records
Find out which weekday a historical or future date lands on.
- Birthday weekdays
- Historic events
- Future planning
Teaching & puzzles
A clear worked example for classrooms and recreational math fans.
- Math lessons
- Algorithm demos
- Brain teasers
Frequently asked questions
What is the Doomsday rule?
It's an algorithm created by mathematician John Conway that finds the day of the week for any date. It relies on a set of memorable dates in each month that always fall on the same weekday within a given year.
What is a 'doomsday' date?
Doomsdays are easy-to-remember dates like 4/4, 6/6, 8/8, 10/10, and 12/12 that all share the same weekday each year. Knowing that shared weekday lets you count to any nearby date.
Does it work for any year?
Yes, for dates in the Gregorian calendar. This tool covers years from 1600 to 2200 and adjusts January and February for leap years automatically.