From the Britannica Online Encyclopedia: The astronomical year, the time taken for the Earth to complete its orbit around the Sun, is about 365.242 days, or, to a first approximation, 365.25 days. To account for the odd quarter day, an extra calendar day is added every four years, as was first done in 46 bc, with the establishment of the Julian calendar. Over many centuries, the difference between the approximate value 0.25 day and the more accurate 0.242 day accumulates significantly. In the Gregorian calendar now in general use, the discrepancy is adjusted by adding the extra day to only those century years exactly divisible by 40(e.g., 1600, 2000). For still more precise reckoning, every year evenly divisible by 4,000 (i.e., 16,000, 24,000, etc.) may be a common (not leap) year.
Mythology that surrounds Leap Year (from Credo):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.