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Meaning of bissextus

bis•sex•tus

Pronunciation: (bī-seks'tus, bi-), [key]
— n.
  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.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.