Today marks the 29th of February, an event that only happens once every four years. It's leap day, an extra day added to the calendar every fourth February to keep our seasons from getting out of whack.
Everyone knows a year is 365-days long, but that's not exactly accurate. It actually takes the Earth about 365 and one-quarter days to travel around the sun. To keep the extra time from throwing things off, the Romans decided a couple thousand years ago to add an extra day every four years.
Pope Gregory the Thirteenth made some slight adjustments in the 1500s to deal with an 11-minute discrepancy. It's now commonly known as the Gregorian calendar and is still followed to this day. In the United States, leap year coincides with presidential election years.