With 2012 being a leap year, it's a good time to remind ourselves why it is that we add an extra day to our calendar every four years. The answer simply put, is that the Earth doesn't take exactly 365 days to orbit the earth. It's actually closer to 365.242 days. While that may not seem like much, it amounts to almost 6 extra hours a year, which does add up over time. A leap year helps to prevent the shift that would happen in our calendars because of these extra hours. Without it our calendars would eventually fall out of sync; in 100 years it would be out by 24 days.
Ancient civilizations had a variety of ways to accommodate this.....after all harvests, rituals and celebrations were all pegged to specific days on a calendar. Many of the accommodations involved complicated maneuvers that often made things worse like adding a whole month to certain years.
The idea of a leap day was introduced in the first century BC by Roman Emperor Julius Caesar who decided that every fourth year would have an extra day to offset the shift. While this was a good plan it wasn't perfect. Given that each year had only an extra .242 of a day, over time, the extra day added every four years also threw the calendar off by one day every 128 years!
In 1582 the Gregorian calendar introduced by Pope Gregory XIII remedied this problem by adopting new rules which we use to this day. To be a leap year the year must be divisible by four. Any year that is evenly divisible by 100 is not a leap year, unless it is also evenly divisible by 400.
And now you know!