Before Julius Caesar reformed the calendar, there wasn't a single leap day. Periodically, if irregularly, multiple days were added to the calendar just before March 1, the Kalends of March. Such insertion of days is called intercalation. With the reform of the calendar, February came to have a regular number of days, with exceptions for the intercalation of a leap day every four years. Gildersleeve's Latin Grammar says the expression for leap day was a.d. bis sextum kal. Mar..
What do those abbreviations mean? See Julian Dates for an explanation.
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