Doomsday Rule

John Conway's Doomsday Rule is a perpetual calendar algorithm; i.e., it is a way to calculate the day of the week of any given date. I prefer it to other methods becuase the calculations are (with some practice) simple enough to do mentally, and it doesn't require memorizing too many things. Here are some notes that I wrote up on the Doomsday Rule for a recreational mathematics course at Michigan Tech. There is a long selection of exercises at end; these are good practice for anyone who wants to become proficient with these calculations. This file is also available in a PDF version.

Here is a Mathematica file that implements the Doomsday Rule as well as Conway's rules for computing Easter and Rosh Hashana.

One advantage of being proficient with the Doomsday Rule is that you can spot calendrical mistakes in books and movies. Here is a short collection of day-of-the-week bloopers that I have noticed.

For another account of the Doomsday Rule, see Rudy Limeback's site.

Conway once told me that he developed the Doomsday Rule when Martin Gardner showed him Lewis Carroll's perpetual calendar algorithm and challenged him to come up with something simpler.

For an extensive discussion of calendar computations, see Claus Tøndering's Calendar FAQ.


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