FEBRUARY 29, known as a LEAP DAY in the Gregorian calendar, is a date that occurs in most years that are evenly divisible by 4, such as 2004, 2008, 2012 and 2016.
Years that are evenly divisible by 100 do not contain a leap day, with the exception of years that are evenly divisible by 400, which do contain a leap day; thus 1900 did not contain a leap day while 2000 did. Years containing a leap day are called leap years.
February 29 is the 60th day of the Gregorian calendar in such a year, with 306 days remaining until the end of that year. Leap years
Although most years of the modern calendar have 365 days, a complete revolution around the sun takes approximately 365 days and 6 hours. Every four years, during which an extra 24 hours have accumulated, one extra day is added to keep the count coordinated with the sun's apparent position.
It is, however, slightly inaccurate to calculate an additional 6 hours each year. A better approximation 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 be 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 13 times on a Sunday, Tuesday, or Thursday; 14 times on a Friday or Saturday; and 15 times on a Monday or Wednesday.
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http://archbishopterry.blogspot.com/2012/02/leap-day-wednesday-in-lent-week-i.html
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