Happy Leap Year day! It's been four years since the last one (2000 had a February 29):

The leap year is a contrivance so that the calendar year (usually 365 days) doesn't get too far away from the solar (astronomical) year. You say: huh? Well, the astronomical year - the time it takes the earth to go exactly once around the sun - is not precisely 365 days. The ancients estimated it as 365* days. That wasn't bad as calculations go; it's actually 365 days, 5 hours, 48 minutes, and 46 seconds.

To prevent this drift between the calendar year and the astronomical (seasonal) year, we add one extra day every four years. Thus, over the four year period, we have 1461 days, not 1460, for an average of 365.25 days per year.

Does that do it? Sadly, no. There are still those extra seconds - the astronomical year is 365.2422 days. So every 400 years, we DON'T NOT add the extra day (double negative intended). So 1700, 1800, 1900 were NOT leap years, but 2000 was.

If you've followed the math, that gets us very close. Over a 400 year period the calendar will contain an average of 365.2425 days per year.

Every 4,000 years (the first will be the year 4000, then 8000, etc.) we make the century years NOT leap years again. And that gives us an average of 365.24225 days per year over a 4.000 year period. Still not exact, but the calendar year won't vary by more than a day from its current place in the seasonal (astronomical) year in two hundred centuries - close enough for practical purposes

(from straightdope.com).