A leap year happens every 4 years when February has 29 days instead of 28. So there are 366 days in a year instead of the usual 365. We do this so the calendar year stays aligned with the astronomical/seasonal year.
Actually, we don't add an extra day every four years. The century year is a leap year only if the number of centuries is evenly divisible by four. The year 2000 was a leap year but 1900 wasn't and 2100 won't be.
9 months ago
Last edited at 7:59PM on 3/1/2013
Almost - a change of century is only a Leap Year if the number is divisible by 400 - as was the year 2000. The year 2100 can't be a Leap Year. This convention was set by the Scribes of Pope Gregory who worked all this stuff out. Medieval science was surprisingly good, because we haven't changed the system with our much greater knowledge today - but we do add or subtract Leap Seconds now to keep our clocks in total synchronisation with the stars. In his time, the seasons were hopelessly out-of-date with the 365-day calendar and the Pope appointed all the clever scientists to work out a solution. Gregory got our calendars back into sync with the weather by subtracting eleven days from the year changes were made. Peasants marched the streets in protest demanding their eleven days back!