Intuitively: Every seven years. Remembering leap years: In 28 years, tops. Thinking about it: Has to be more often than that; there are only seven weekdays the

Intuitively: Every seven years. Remembering leap years: In 28 years, tops. Thinking about it: Has to be more often than that; there are only seven weekdays the year can begin on. This is only considering getting the weekday for every date right; moon-phase stuff like the date of Easter will make it (much?) more rarely. OK, will go read the article now...