A mental method, invented by John Conway, for finding the day of the week of any date in the Gregorian calendar — in your head, in seconds.

Give it any date — a birthday, a historical event, a date in 2087 — and it returns Monday, Tuesday, and so on, with no lookup and no memorised calendar. It rests on one clever anchor: each year has a special weekday (its "doomsday") that a set of easy dates always fall on, so every other date is a short count away. It's the classic "what day were you born?" party trick.

As numbers 0–6, starting from Sunday — so all the arithmetic is done mod 7.

Every step of Doomsday adds or subtracts small numbers and reduces mod 7 (keep the remainder after dividing by 7). A result of 4 means Thursday. The whole trick is arithmetic on a 7-hour clock.