Leap Year in French – année bissextile
It was Julius Caesar who introduced the Leap Day, in order to bring the various religious calendars back into alignment.
As we already know from Roman numerals, numbering in sequence, as we know it today, was not a Roman habit.
After the Julian reforms, the Romans inserted another day between February 23 and 24, and called it “Bis VI Kal. Mart.” (Second sixth day before the First of March.)
When the days of the month were numbered in sequence in the Middle Ages, the day was moved to the last day of February, but the term bissextile remained.
Bis in French describes something that happens twice.
eg J’Habite à 5 bis, rue de Perpignan. – I live at 5 A
un itinéraire bis – detour, diversion