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bissextile (by-SEKS-til) adjective

Of or pertaining to the leap year or the extra day in the leap year.

noun

Leap year.

[From Late Latin bisextilis annus (leap year), from Latin bissextus (February
29: leap day), from bi- (two) sextus (sixth), from the fact that the sixth
day before the Calends of March (February 24) appeared twice every leap year
or perhaps because 366 has two sixes.]

"Do people born on Feb. 29 celebrate their birthdays on Feb. 28 or March
1 in non-leap years? The last day of February is the natural and logical
choice. A term for leap year, `bissextile,' means doubled day - referring
to a double Feb. 28."
Happy Birthday Leap Year Babies, The Toronto Star (Canada), Feb 26, 1999.