This Day in World History
October 15, 1582
Use of Gregorian Calendar Begins
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. This shift caused problems for the Roman Catholic Church because it affected the date of Easter.
To fix that problem, Pope Gregory XIII instituted a calendar reform that eliminated the leap day in century years not evenly divisible by four. Thus 1600 and 2000 have leap days but not 1700, 1800, or 1900. This fix meant the calendar would be accurate for thousands of years. There still remained the problem of the season creep, however. The solution to that was straightforward. The pope eliminated 10 days from October 1582. October 4, then, was followed by October 15.
Gregory’s edict instituting the calendar was carried out in Roman Catholic countries throughout Europe, but the reform was not immediately adopted in Protestant countries. Protestant states in Germany did not accept the new calendar until 1699, and Great Britain did not adopt until 1752. The Julian calendar persisted throughout Easter Orthodox lands into the twentieth century and is still used to determine the dates of religious holidays. Russia did not adopt the Gregorian calendar until 1918. This is why the October Revolution in 1917 that brought the Bolsheviks to power actually took place in November.