Most people know that there's a Leap Year every four years so the calendar can stay on track with the seasons. What I didn't know is that since the Earth takes a little less than a quarter day per year extra, adding an extra day every four years would still make us lose ground to the seasons. Because of this, every hundred years, there is no Leap Year at the turn of the century. So there's a Leap Year in 1796 and 1804, but not in 1800. HOWEVER, this adjustment still isn't enough to keep the calendars right, so every 400 years (or century years that are divisible by 400) there IS a Leap Year. This explains why 2000 was a Leap Year, but 1900 was not.

Think of all the knowledge 30 Rock could have mixed into their Leap Day episode last week.

UPDATE:
Ooooh. This video explains it much better than I did.