bissextile (by-SEKS-til) adjective
Of or pertaining to the leap year or the extra day in the leap year.
noun
Leap year.
[From 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.]
-Anu Garg (words at wordsmith.org)