So-called since the 1400s because anniversaries or fixed feast days after February in such a year, leap or jump by two days not one. If one’s birthday, for example, fell on a Monday in the year preceding a Leap Year, it would fall on a Wednesday in the following year instead of the Tuesday, thus ‘leaping’ a day. The extra day added to the Julian calendar at the end of February every four years started in 46 BC. It makes up the difference between the solar year and the calendar year. The Earth actually takes 365.242 days to travel around the sun. Rounded off to 365.25, the discrepancy is corrected every four years by the addition of the extra day.