Leap Year:
Because the Romans referred to dates according to their relationship with the named days, the sixth day before the first day of March was called diem sextum Kalendas Martias ("the sixth day before the calends of March")
Julius Caesar created the Julian calendar, which replaced the extra month with an extra day in February. 'Bissextile day' comes from the Latin 'bissextus' and means the "second sixth day" or "double sixth day.". A leap year is a year that has an extra day—366 days, with February 29 as the extra day. It has another name in English: bissextile year (and leap day is also known as bissextile day).
Congratulations to Smitty03275 and evilfoot for the first and second to find.