In ancient Rome February 24 was doubled in a leap year making it 48 hours long, in the same way that February 28 is doubled every four years, except we call it a different day.
All leap days and years are attempts to to align a calendar, which is based on whole days and the orbit of the Earth around the Sun, which cannot be nicely divided into 24-hour days. Using a purely day-based calendar, Earth’s exact position in its orbit around the Sun would change slightly in date each year. To make up for this slight difference Ancient Rom made February 24 48 hours long every four years. The day February 24 was chosen because it was the day after the flight of the last King of Rome, Lucius Tarquinius Superbus, on February 23. Since our calendar (the Gregorian, 1582) is based on the Julian (46 B.C.) the tradition of adding an extra day in February as remained.
This image Fasti Antiates Maiores is a miniature black and white image of a 1 m high by 2.5 m wide fragmentary fresco of a pre-Julian Roman calendar found in the ruins of Nero’s villa at Antium (Anzio).