It takes Earth 365.25636 days to complete its orbit around the sun. It is called a "sidereal year." Because it's pretty silly to have a fraction of a day, we humans created the Julian calendar, named after Julius Ceasar who introduced it in 46 BC and rounded the year down to 365.25 days. Also called the "western calendar," that pesky fraction of a day problem was resolved by adding an extra day every 4 years. We call these special years "leap years" and observe them on February 29th, like today! But because 365.25 does not equal 365.25636, we also had to create the Gregorian calendar in 1582 to account for the accumulated error in our 365.25 day approximation by skipping 3 leap years every 400 years. What a mess...