February 29th is a day that only appears on calendars once every four years, which we call Leap Year. It makes up for the fact that a year is actually 365.25 days long. Why do we put that extra day in February? I used to think it was because February is the shortest month, but that raises the question of why February got shortchanged in the first place. The simple answer is because in ancient Rome, the calendar year began with March. That's why October has a name that means eight even though it's the tenth month. In the Roman calendar, February was the final month of the year, so the placement of Leap Day at the very end of the calendar makes sense.
Except the Romans had Leap Day on February 24th. They essentially had two February 24ths every four years, which was all kinds of confusing, especially if you had an appointment that day. You can thank medieval monks for adding a new day to the calendar, which had to be kept in February because of the calculations of the spring equinox. Read how all that came about at the Conversation. -via Damn Interesting