bissextus [bahy-seks-tuh s, bi-]
1. 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.
Origin: < Late Latin bissextus (diēs) intercalary (day); see bis, sext; so called because the 6th day before the Calends of March (Feb. 24th) appeared twice every leap year
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