When was the Gregorian calendar introduced?

The Julian calendar, introduced by Julius Caesar in 46 BCE, assumed a year of 365.25 days, which was about 11 minutes too long. Over centuries this error had pushed the calendar about 10 days out of line with the seasons. To fix it, Pope Gregory XIII decreed that Thursday 4 October 1582 would be followed by Friday 15 October 1582. The new system also refined leap years: century years are leap years only if divisible by 400. Catholic countries adopted it at once, but Britain and its colonies waited until 1752, and Russia until 1918.