What day of the week was it?
John Conway's Doomsday method lets you name the weekday of any date in your head. Every year has a "doomsday", the weekday shared by 4/4, 6/6, 8/8, 10/10, 12/12, 5/9, 9/5, 7/11, 11/7 and the last day of February. Press Reveal on a date to see the six steps that find it, then count to your date from the nearest doomsday. When you have the hang of it, try Quiz me.
Reveal the steps
Quiz me
What's going on
The whole calendar repeats with a few simple facts. A normal year is 52 weeks and 1 day, so each year the doomsday moves forward one weekday, and a leap year pushes it two. That is where the "year + year ÷ 4" count comes from. The century anchors (Tuesday for the 2000s, Wednesday for the 1900s) are just the starting points.
The doomsday dates are chosen to be memorable: the even months from April on pair with themselves, and "I work 9 to 5 at the 7-Eleven" (9/5, 5/9, 7/11, 11/7) covers May, September, July and November. The last day of February is the anchor for the start of the year.
With a little practice people do this in under ten seconds. Conway could do it in two.