Leap Year (Celebrate Today)

ONE OF the convenient fictions we live by holds that there are exactly 365 days in a year. In point of fact, the earth turns roughly 365 and a quarter times on its axis by the time it has completed a full year's orbit around the sun, which means that periodically the calendar has to catch up, thus the convention of leap years. A leap year contains one extra day, February 29, for a total of 366 days. 2012 is a leap year.
So, where does the "leap" come in? This is a perennial source of confusion. In a normal sequence of years, a calendar date that falls on, say, a Monday one year will fall on Tuesday the next, Wednesday the year after that, Thursday the year after that, and so on. But every fourth year, thanks to the extra day in February, we "leap" over the expected day of the week — Friday, in this case — and that same calendar date lands on Saturday instead.
Even more abstruse is the arithmetical formula used to calculate which years are leap years, here described as succinctly as one could ever hope by Rev. E. Cobham Brewer, author of Brewer's Dictionary of Phrase and Fable:

[A leap year is] any year whose date is exactly divisible by 4 except those which are divisible by 100 but not 400.

Why such complexity? Because the exact number of days in a solar year is actually ever-so-slightly less than 365.25 (it is 365.242374, to be precise), so the algorithm had to be designed such that every now and then a leap year is skipped to keep the calendar on track over the long haul.