February
From circa 700 BC, when Numa Pompilius, the second king of Rome, added it to the calendar, February had 23 days and 24 days on some of every second year, until 46 BC when Julius Caesar assigned it 29 days on every fourth year and 28 days otherwise. Leap year Day, February 29, is added in every year that can be divided equally by four, such as 2012 and 2016, but this does not apply when the year ending in "00" at the turn of the century does not divide equally into 400. This means that 1600 and 2000 were leap years in the Gregorian calendar, but 1700, 1800, and 1900 were rather common years. This is where the Julian calendar calculated dates differently, as it always repeated February 29 every four years.