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From the Astronomical Dictionary

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the bissextile day

bissext- = [Latin] the second sixth

A bissextile day is a day that is inserted every few years to keep the calendar in line with the tropical year, of which the average length is not a whole number of days. In the Gregorian calendar the date of 29 February occurs only in a leap year. This date is nowadays seen as the extra day of a leap year, but in the original Julian calendar as installed about two thousand years ago by the Romans the 23rd day of February was doubled, and the last day of February had the same name in a leap year as in a non-leap year. The Romans referred to the 23rd day of February as the 6th day before the beginning of March, and the extra inserted day was the "second sixth" day, or bissextile day.