20 July 1969? Sunday. With three small ideas and a little practice you can work out the weekday of any date from 1583 to 2400 — entirely in your head, in seconds.
Learn is a guided, animated walkthrough of John Conway's Doomsday algorithm — seven short lessons, each with things to play with, ending in a playground where you can watch the full derivation for any date.
Train is a timed drill: you're shown a random date, the clock runs, and you pick the day. Get it wrong and you're shown a complete step-by-step debrief of how that exact date should have been worked out.
Seven short lessons. Work through them in order — each one adds a single idea.