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

I recall that Tom and Ray Magliozzi labeled these days anchor days.

This happens because the difference in days between the anchor days 4/4, 6/6, 8/8, 10/10, and 12/12 is 63, a multiple of 7, i.e., a full week. That in turn is because the two ends of months included are a pair of months with 30 and 31 days.

Interesting!

As an anchor day for March, I recommend pi day. January is on the previous year’s doomsday– the second might be the most memorable

My birthday falls on thursday this year! So i can follow the 4/4 6/6 8/8 10/10 and 12/12 method to see when my birthday falls on that year mine is 11/21 in case you were wondering btw