Leap day is a term that we often use to refer to February 29th, a day that happens once every four years to keep our calendars in sync with the Earth's rotation around the sun. However, there are a number of synonyms for leap day that you may not be familiar with. For example, some people may call February 29th a "leap year day" or a "bisextile day." Others may refer to it as a "bonus day" or a "surplus day." Regardless of what you call it, the extra 24 hours this day provides serves a valuable purpose in keeping our calendars accurate.