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What the Day teaches the Doomsday algorithm — a mental-math method, devised by the mathematician John Conway, for working out which weekday any date falls on, without a calendar or a phone. Give it a little practice and you can answer “what day was 4 July 1776?” in a few seconds.
Most explanations of the Doomsday algorithm are a single dense page of formulas. That is enough to understand it, but not enough to get fast at it. Real fluency comes from drilling each small step until it becomes automatic — so this site breaks the method into a step-by-step course and a set of focused practice games, then lets you put it all together in play mode and a shared daily challenge.
What the Day is built and maintained by Jesper Kiledal as an independent side project. It is free to use. If you would like to support it, you can buy me a coffee; the site also shows a few ads to help cover its running costs.
What the Day is a Progressive Web App: you can install it from your browser menu (or the prompt on the play page) for a full-screen, home-screen app on your phone or desktop. Normal play mode works without an internet connection — dates are generated on your device, and if you are signed in your results are stored locally and synced to your account automatically the next time you are online. Learning pages and guides you have already read stay available offline too.
Questions, ideas, or found a bug? Head to the contact page — feedback is always welcome. You can read how your data is handled in the privacy policy.