Why does February have 29 days every 4 years.
A leap year occurs every four years to help synchronize the calendar year with the solar year, or the length of time it takes the earth to complete its orbit about the sun, which is about 365¼ days.
The length of the solar year, however, is slightly less than 365¼ days, by about 11 minutes. To compensate for this discrepancy, the leap year is omitted three times every four hundred years.
In other words, a century year cannot be a leap year unless it is divisible by 400. Thus 1700, 1800, and 1900 were not leap years, but 1600 and 2000 were.
—The Fact Monster