Bissextus: (or double sixth): 1. February 29th: the extra day added to the Julian calendar every fourth year (except those evenly divisible by 400) to compensate for the approximately six hours a year by which the common year of 365 days falls short of the solar year. ety: Bissextus comes from the Latin term bissextus diēs meaning "intercalary day." It was so called because the 6th day before the Calends of March (February 24th) appeared twice every leap year. This odd day was inserted after the sixth day before the kalends of March, i.e., after the 24th of February, and was not counted as an addition to the year, but as a sort of appendix. Hence the sixth of the kalends of March was called bissextus, or double sixth, which root is still retained in our word bissextile, though the day is now added at the end of February.
In 46 BC, Julius Caesar created a calendar system that added one leap day every four years. Acting on advice by Alexandrian astronomer Sosigenes, Caesar did this to make up for the fact that the Earth's year is slightly more than 365 days, the time it takes for the Earth to circle the Sun is slightly more than the time it takes for the Earth to rotate 365 times (with respect to the Sun -- actually we now know this takes about 365.24219 rotations). So, if calendar years contained 365 days they would drift from the actual year by about 1 day every 4 years. By adopting a leap year with an extra day every four years, the calendar year would drift much less. This Julian Calendar system was used until the year 1582 when Pope Gregory XIII provided further fine-tuning when he added that leap days should not occur in years ending in "00", unless divisible by 400. This Gregorian Calendar system is the one in common use today.