February 29th: the extra day added to the Julian calendar every fourth year (except those evenly divisible by 400) to compensate for the approximately six hours a year by which the common year of 365 days falls short of the solar year
Word origin
[‹ LL bissextus (diēs) intercalary (day); see bis1, sext; so called because in the original Julian calendar the 6th day before the Calendsof March (Feb. 24th) appeared twice every leap year]