Word of the Day.

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)