Bissext, or bissextus (Lat. bis, twice; sextus, sixth), the day intercalated by the Julian calendar in the February of every fourth year to make up the six hours by which the solar year was computed to exceed the year of 365 days. The day was inserted after 24 February, i.e. the sixth day before the calends (1st) of March; there was consequently, besides the sextus, or sixth before the calends, the bis-sextus or "second sixth," our 25 February. In modern usage, with the exception of ecclesiastical calendars, the intercalary day is added for convenience at the end of the month, and years in which February has twenty-nine days are called "bissextile," or leap years.

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