Doomsday rule

The Doomsday rule, also called the Doomsday algorithm, is a method for determining the day of the week for any given date. It was devised by the mathematician John Conway in 1973, drawing on inspiration from a perpetual calendar algorithm created by Lewis Carroll. The method works because the Gregorian calendar repeats in 400-year cycles and because each year has a set of easily memorized dates, called doomsdays, that always fall on the same day of the week. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/