Wikipedia
Bissext, or bissextus ( Lat. bis, twice; sextus, sixth) is the day which is added to the Gregorian calendar every fourth year (except those evenly divisible by 100, unless they are divisible by 400) to compensate for the six-hour difference in length between the common 365 day year and the actual length of the solar year.
Originally, the day was inserted after the 24th of February, i.e. the 6th day before the calends (1st) of March, Consequently, besides the sextus, or sixth before the calends, the bis-sextus or "second sixth," was our 25 February. In modern usage, with the exception of ecclesiastical calendars, the intercalary day is added for convenience at the end of the month of February, as February 29, and years in which February has twenty-nine days are called "bissextile," or leap years.