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bissextus

[bahy-seks-tuh s, bi-]
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noun
  1. February 29th: the extra day added to the Julian calendar every fourth year (except those evenly divisible 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.

Origin of bissextus

< Late Latin bissextus (diēs) intercalary (day); see bis1, sext1; so called because the 6th day before the Calends of March (Feb. 24th) appeared twice every leap year
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018