John Conway’s “Doomsday” rule observes that that every year, the dates 4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, and 11/7 fall on the same day of the week. In 2024 all these dates fall on a Thursday.

These dates are easy to memorize because they break down in double pairs of even digits—4/4, 6/6, 8/8, 10/10, and 12/12— and symmetric pairs of odd digits—5/9 and 9/5, 7/11 and 11/7. Because of their symmetry, this set of dates is the same whether interpreted according to the American or European convention.

In addition, the last day of February falls on “Doomsday.” Since 2024 is a leap year, this will be February 29.

Related post: Mentally calculating the day of the week

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