The Doomsday rule: mental day-of-week calculations
davecturner.github.io
The Doomsday rule is an algorithm for working out the day of the week of a given date. It’s based on John Conway’s observation that certain memorable dates called doomsdays (4/4, 6/6, 8/8, 10/10, 12/12, 9/5, 5/9, 7/11, 11/7, …) always occur on the same day of the week in any given year. This day is known as the year’s anchor day. To compute an arbitary day of week you work out the anchor day for the given year and the offset between the target date and an appropriate doomsday. It’s intended to be simple enough that you can do the computation in your head with a bit of practice.
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The original was posted on /r/todayilearned by /u/Kthulu71 on 2024-12-18 22:52:46+00:00.

Original Title: TIL about the oddly-named “Doomsday Rule” which can (with practice) be used to calculate the weekday of a target date based on the fact that certain days (4/4, 5/9, 6/6…) in any given year all fall on the same day of the week.