Free interactive game for learning Conway's Doomsday Algorithm — the mental math method to calculate the day of the week for any date in your head. Timed practice, heptagon layout, streak tracking, global leaderboard.
The Doomsday Algorithm, invented by mathematician John Horton Conway, lets you mentally calculate the day of the week for any date. It uses "doomsday" anchor dates that always fall on the same weekday within a year.
4/4, 6/6, 8/8, 10/10, 12/12 — May 9, September 5, July 11, November 7 ("9-to-5 at 7-Eleven") — Last day of February — January 3 (or 4 in leap years).
1800s: Friday — 1900s: Wednesday — 2000s: Tuesday — 2100s: Sunday.