The Julian year (introduced by Julius Caesar in 45 B.C.E.) contained exactly 365.25 days and had a leap year every fourth year (when the year number was divisible by four). But the actual year is 365.24219879 days long and thus the Roman calendar gradually became out of step with reality. By the sixteenth century, the calendar was ten days adrift from the seasons.
In 1576 Pope Gregory XIII assembled a commission of astronomers, mathematicians, and clergy, and this advisory body eventually adopted a plan suggested by the Calabrian physician Luigi Lilio, who was also known as Aloysius Lilius. On February 24, 1582, the pope declared that Thursday, October 4, 1582, was to be followed by Friday October 15. From that date, century leap years would only be allowed if the year number was exactly divisible by 400. Thus, 2000 was a leap year, to be followed by 2400,2800, and so on.
Roman Catholic countries immediately adopted the Gregorian Calendar, but the Protestant and Greek Orthodox countries would have none of it. Norway and Denmark finally changed their minds in 1700. Great Britain and its colonies, including the United States, eventually followed, in 1752. In that year, in those territories the day after September 2 was September 14. Japan changed to the Gregorian calendar in 1873, Russia adopted it in 1918 after the Revolution, and Greece waited until 1923.
There are still minor problems. By 13000 the calendar will be ten days out of step again. And the fact that the Earth's spin rate is slowly decreasing in a complicated fashion means that the system should be subtracting a day about every 2,000 years.
