Use of Gregorian calendar begins
This Day in World History – In Roman times, Julius Caesar instituted a calendar reform based on a solar year of 365 and one-quarter days. To accommodate the quarter day, the Julian calendar added an extra day to every fourth year, creating leap years. Unfortunately, a solar year is really a few minutes shorter than 365 days and 6 hours. The Julian calendar’s overestimate meant that over the course of a century, more or less, the beginning of each of the four seasons moved back a day. By the late 1500s, the spring equinox fell on March 11, rather than around March 21.