Mark this one on your calendar — “bissextile” is a fancy word to describe a leap year. The word “bissextile” comes from Latin, with “bis” meaning “twice” and “sextus” meaning “sixth.” When the calendar reads February 29, the word “sixth” might not make much sense, but under the Julian calendar, the extra day was the sixth day before the beginning of March. We have to add this extra day because the Earth travels around the sun in 365 days and a little bit less than a quarter of a full day. Throughout history, various calendars either eventually lagged behind, or added in extra random dates to catch up. Under the current Gregorian calendar, the extra time is accounted for by inserting an extra day into February in the bissextile year (every four years). Since it is just a hair less than a full quarter of a day, the Gregorian calendar accounts for this with one more rule. Centenary years are bissextile years only if they are divisible by 400. (For example, 2000 had a February 29, but 2100, 2200, and 2300 will not.)