Ultraviolet, 2 years ago

The Doomsday rule.

Not necessarily the part for calculating the day of the week for any arbitrary day centuries ago, that’s just a useless party trick, but for the current year so you don’t need to pull out your phone to check. Knowing that 1/3 (or 1/4 on a leap year), the last day of February, 3/14, 4/4, 5/9, 6/6, 7/11, 8/8, 9/5, 10/10, 11/7, and 12/12 are all the same day of the week, that this year they’re all Tuesdays, and next year they’re all Thursdays, is mostly easy to remember and very frequently useful.