bissextus

[ bih-seks-tuhs, bee- ]

noun

1. Also bissext. February 29th: the extra day added to the Julian calendar every fourth year (except those evenly divisible by 100, but not by 400, a rule introduced by the Gregorian calendar) to compensate for the approximately six hours a year by which the common year of 365 days falls short of the solar year.

Word History and Origins

Origin of bissextus¹

From Late Latin bissextus (diēs) “intercalary (day)”; so called because the 6th day before the calends of March (Feb. 24th) appeared a second time once every leap year (hence the “second sixth”); bis ¹, sext ¹