From the word bissextus, which refers to February 29th.
Bissextus means literally “the second sixth,” so called because the 6th day before the first day of March in the ancient Roman calendar appeared a second time once every leap year.
EXAMPLES OF BISSEXTILE
The extra day in a bissextile helps keep the calendar in sync with the solar year.
February 29th is an occurrence that only happens in a bissextile.