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Keep practicing until you can find the correct weekday for any date quickly and consistently.
Confidently determine the weekday of any given date, anywhere and anytime without relying on a calendar.
Sharpen your mental agility by mastering the art of turning a date into a weekday.
Be the one who casually answers when someone asks what day Christmas lands on this year.
The perpetual calendar you keep in your head
Soon you won’t need us to tell you that.
John Conway’s Doomsday rule names the weekday of any date — past or future — in your head, in seconds. Learn it here in an afternoon. Free, no account, seven languages.
However you like to learn — read it, drill it, or just play.
Both players get the same date. Fastest correct answer wins, and your rating moves — climb from Bronze to the Doomsday tier. Every Battle is a minute of pure mental math against a human who wants your points.
It’s a method for working out the day of the week for any date — past or future — in your head, in seconds. The trick is that a handful of memorable dates each year all share the same weekday, called that year’s doomsday. Find it, and every other date is a short count away.
Try it with your own birthday below, then learn the full method.
This isn’t a knack you’re born with. It’s a skill that gets measurably faster every week you practice — until the weekday just appears the moment you read a date.
See how fast you are