The Julian calendar, which was introduced in 45 BC by Julius Caesar. Based, in theory, on a solar year (the time it takes the Earth to orbit the sun – actually 365 days, 5 hours, 48 minutes, 46 seconds) it established a cycle of three years of 365 days, followed by a 366-day leap year.
Why the need for a change?
Because the Julian calendar had too many leap years. Having one every four years slightly overcompensated for the actual length of a solar year, leading to an error of one day every 128 years. As a result, by 1582 the dates of seasonal equinoxes were falling 10 days too early, affecting the computation of the date of Easter which is based on the vernal (spring) equinox.
What was done about it?
To bring things back into line, Pope Gregory XIII issued the Papal Bull ‘Inter Gravissimus’ in 1582 establishing what became known as the Gregorian calendar. Ten days were dropped from the month of October 1582, and from then on century years (eg 1600) could only be leap years if they were divisible by 400.
Did this do the trick?
Almost. The Gregorian calendar is still slightly inaccurate, but only by a day every 3,236 years.
Was the new calendar universally accepted?
Catholic countries adopted the new calendar almost immediately but non-Catholic ones weren’t going to have a pope tell them what to do and took a lot longer to adopt it. Parts of Germany, for example, only made the change in 1698 and Greece waited until 1923. (The Islamic calendar is rather different because it is based on the movement of the moon, not the sun, leading to years of 354–355 days.)
What about Britain?
Britain finally adopted the new calendar in 1752 – and also switched the start of the new year from 25 March to 1 January. By that time the difference between the two calendars was 11 days. To solve the problem, in that year, 2 September was followed by 14 September. Reports that the change was greeted by rioting mobs in the streets chanting “give us back our 11 days!” appear in fact to be without foundation.