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Master the<br>Doomsday Algorithm<br>Find the weekday forJuly 20, 1969

in seconds. It's a... thinking ...

Play now<br>Daily challenge<br>Practice

Quick Challenge<br>Find the weekday for:<br>July 20, 1969

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Challenge yourself<br>Keep practicing how quickly and consistently can you find the correct weekday for a date.

Apply anywhere<br>Confidently determine the weekday of any given date, anywhere and anytime without relying on a calendar.

Stimulate the brain<br>Enhance your mental agility by mastering the art of date-to-weekday conversion.

Impress your friends<br>Be the one who casually answers when someone asks what day Christmas lands on this year.

The transformationFrom counting it out<br>to instant recall<br>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.<br>See how fast you are<br>Beginnercounting through the rules20–30s/ date

Intermediateanchors memorized5–10s/ date

Expertpure recall/ date

244,700+ dates solved<br>1.03s fastest on record<br>8 ranks to climb

Where to start<br>However you like to learn — read it, drill it, or just play.

Learn the algorithm<br>A step-by-step course: doomsdays, anchor days, leap years and more.<br>Open<br>Practice games<br>Drill each sub-skill as a fast, focused mini-game.<br>Open<br>Play<br>Find the weekday for random and historical dates.<br>Open<br>Daily challenge<br>The same five dates as everyone else, every 24 hours.<br>Open<br>Guides<br>Long-form articles, memory tricks and a practice plan.<br>Open

What is the Doomsday algorithm?<br>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.<br>Try it with your own birthday below, then learn the full method.

Select a date(like your birthday)<br>You can type it or use the calendar selector

Learn the algorithm

Find this useful?<br>What the Day is a side project I’ve built to help people master the Doomsday algorithm. If you’ve enjoyed using it, consider supporting its development!<br>☕Buy me a coffee