The Doomsday algorithm, devised by mathematician John Conway, rests on one idea: within any year, a handful of easy-to-remember dates all fall on the same weekday— the year’s “doomsday.” Once you know that day, every other date is a short count away.
The course below builds that skill one piece at a time. Work through it in order, and play the linked practice game at the end of each part to make it stick. Prefer to read it as one continuous article? Start with What is the Doomsday algorithm?
The memorable dates that fall on the same weekday every year — the idea the whole method is built on.
Turn weekdays into numbers (0–6) so you can add and subtract them with modulo 7.
The single weekday per century that anchors every year in it.
Spot leap years quickly — they shift the doomsday dates in January and February.
Combine the anchor day and the year to find any year's doomsday.
Use the doomsday to count to the weekday for any date in the year.
A quick taste of what you’ll be able to do.