DOOMSDAY

The Doomsday algorithm, invented by mathematician John Conway, lets you name the day of the week for any date — past or future — entirely in your head, usually in a few seconds. It's the ultimate mental-math party trick: "What day were you born? …That was a Tuesday."

The idea is beautifully simple. Every year has one weekday called its "doomsday," and a handful of easy-to-remember dates (4/4, 6/6, 8/8, 10/10, 12/12, and a few more) always land on it. Once you know a year's doomsday, any date is just a short hop away, counted in sevens.

The whole method is three steps:

1. Century anchor — a memorised day for each century (2000s → Tuesday).
2. Year's doomsday — a small calculation from the last two digits.
3. The date — count from the nearest doomsday date to your target, mod 7.

Four missions:

1. What is Doomsday — the idea, weekdays as numbers, the doomsday concept.
2. The doomsday dates — the memorable dates that share the doomsday, and their mnemonics.
3. Year & century anchors — the century days and the last-two-digits calculation.
4. Full method & practice — putting it together end-to-end, plus leap years and tips.

It pairs naturally with Mental Math (it's pure modular arithmetic) and the Major System (for memorising the anchors). Every date in this module has been checked against the real calendar.