What is a year's "doomsday"?

A single weekday, specific to that year, on which a whole set of easy-to-remember dates all fall.

For example, in 2024 the doomsday is Thursday, so 4/4, 6/6, 8/8, 10/10, 12/12 (and several more) were all Thursdays that year. The genius is that once you know one year's doomsday, you instantly know the weekday of a dozen scattered dates — and any other date is just a short hop from the nearest of them.

Key insight: the special dates are chosen so they always share the year's doomsday.