February is the secon month of the year in the Julian and Gregorian calendars. It is the shortest month and the only month with fewer than 30 days. The month has 29 days in leap years, when the year number is divisible by four (except for years that are divisible by 100 and not by 400 in the Gregorian calendar). In common years the month has 28 days. Some believe that February originally had 29 days (30 in a leap year), but that idea was invented by Sacrobosco during the Middle Ages. See Month lengths.