February 29

I can't let today pass without commenting on its unusual nature. February 29 occurs only once every four years, more or less. We live on the Gregorian calendar, the actual occurance of these extra days follows this rule. 

An astronomical year lasts slightly less than 365 1/4 days. (snip) Each leap year has 366 days instead of 365. This extra leap day occurs in each year that is a multiple of 4, except for years evenly divisible by 100 but not by 400.

As a result of which we manage to keep the seasons in sync with the calendar, the purpose of making adjustments to the calendar.