"The earth actually takes longer than 365 days to complete its trip around the sun—five hours, 48 minutes, and 45 seconds longer, to be precise. To accommodate this discrepancy, an extra day is added to the Gregorian calendar at the end of February every four years (but not in “century” years unless evenly divisible by 400, e.g., 1600 and 2000, but not 1700).
The year in which this occurs is called Leap Year, probably because the English courts did not always recognize February 29, and the date was often 'leaped over' in the records. There's an old tradition that women could propose marriage to men during Leap Year. The men had to pay a forfeit if they refused. It is for this reason that February 29 is sometimes referred to as Ladies' Day or Bachelors' Day. Leap Year Day is also St. Oswald's Day, named after the 10th-century archbishop of York, who died on February 29, 992."