We are all taught in school that there are 365 days in a year and that February is the shortest month of the year with only 28 days in it.
Except on Leap years which occur every four years, on average, but is omitted three times every four hundred years.
On those years there are 366 days to the year and February gains a day at the end to make a February 29th on the calendar.
Infoplease explains the reason behind having a Leap Year.
The Gregorian calendar, which now serves as the standard calendar for civil use throughout the world, has both common years and leap years. A common year has 365 days and a leap year 366 days, with the extra, or intercalary, day designated as February 29. 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.
Let me preemptively, by one day, say Happy Birthday to Leaplings aka Leapers, who had the luck to be born on Leap Day in a Leap Year.