About What the Day
A free web app for learning to find the day of the week for any date — in your head.
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.
Why it exists
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.
Who makes it
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.
Get in touch
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.