Bissextus
Bissext, or bissextus (from Latin bis 'twice' and sextus 'sixth') is the leap day which is added to the Julian calendar every fourth year and to the Gregorian calendar almost every fourth year to compensate for the almost six hour difference in length between a common calendar year of 365 days and the average length of the solar year. In the ancient Julian calendar, 24 February – ante diem sextum Kalendas Martias, the 6th day before the calends, or 1 March, counting backwards inclusively in the Roman style (1/3, 28/2, 27/2, 26/2, 25/2, 24/2) – was doubled in a leap year.