Errors in the Julian Calendar

In 46BC Julis Caesar determined that an extra day should be added to the calendar every fourth year. (To start things off properly, the year now known as 46BC had 445 days.) However, this scheme does not give a completely satisfactory approximation to the actual length of the year and by the 16th Century the accumulated error was becoming noticeable and Easter and the other religious festivals were falling in the wrong part of the year.

Pope Gregory X!!! therefore ordered that 10 days should be dropped from the year 1582 and that henceforth years divisible by 100 but not by 400 should NOT be leap years. The Protestant countries of Northern Europe (and their colonies in, for example, North America) regarded this as a Popish plot and refused to fall into line. For example, Newton's birthday is 4th January 1643 in the Gregorian calendar, but he was born on Christmas Day 1642! However, in 1752 Great Britain adopted the Gregorian calendar -- by which time it was necessary to drop 11 days and so 14th September 1752 followed 2nd September. There were riots in the streets with people demanding "Give us back our 11 days!"

Some other countries were even later adopting the new system: Bulgaria in 1916, the Soviet Union in 1918 and the Greek Orthodox Church in 1924. This is a source of irritation to historians of the period where the two different calendars overlap.