/buy seks"teuhs, bi-/ , n.
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.
[ bissextus ( dies ) intercalary (day); see BIS 1 , SEXT; so called because the 6th day before the Calends of March (Feb. 24th) appeared twice every leap year ]