Doomsday rule

The Doomsday rule or Doomsday algorithm is a way of calculating the day of the week of a given date. It provides a perpetual calendar because the Gregorian calendar moves in cycles of 400 years.

This algorithm for mental calculation was devised by John Conway after drawing inspiration from Lewis Carroll's work on a perpetual calendar algorithm. It takes advantage of each year having a certain day of the week (the doomsday) upon which certain easy-to-remember dates fall; for example, 4/4, 6/6, 8/8, 10/10, 12/12, and the last day of February all occur on the same day of the week in any given year. Applying the Doomsday algorithm involves three steps: