Leap Day
It is, however, slightly inaccurate to calculate an additional 6 hours each year. A better approximation, derived from the Alfonsine tables, is that the Earth makes a complete revolution around the sun in 365 days, 5 hours, 49 minutes, and 16 seconds. To compensate for the difference, an end-of-century year is not a leap year unless it is also exactly divisible by 400. This means that the years 1600 and 2000 were leap years, as will be 2400 and 2800, but the years 1700, 1800 and 1900 were not, nor will 2100, 2200 and 2300.
The Gregorian calendar repeats itself every 400 years, which is exactly 20,871 weeks including 97 leap days. Over this period, February 29 falls on Sunday, Tuesday, and Thursday 13 times each; 14 times each on Friday and Saturday; and 15 times each on Monday and Wednesday.
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