The Gregorian calendar is also sometimes called the ‘Christian calendar’ or the ‘Western calendar’.
It is the most common calendar in general use and accepted internationally as the civil calendar, which means that it is used for official and administrative purposes - although in some countries and cultures, other calendars are also employed for non-official purposes. (Although the calendar was originally introduced by the Catholic church, it is now also used by non-Catholic and non-Christian countries out of convenience.)
It is named after Pope Gregory XIII, who introduced it.
The decree introducing the new calendar was signed by Gregory on 24th February 1582.
It was designed to replace the Julian calendar, which was introduced by Julius Caesar in 45 BC.
The problem with the Julian calendar is that it miscalculates the length of a year by 11 minutes. This isn’t very important over a short period of time, but over decades and centuries, the accumulated extra minutes each year begin to add up. By the time of Pope Gregory, the error added up to around 10 days.
The error with the Julian calendar concerned Gregory and other members of the Catholic church because it meant that the date of the vernal (Spring) equinox was effectively shifting and the Catholic Church used the equinox date to calculate the date of Easter, which is the most important day in the Christian calendar.
The number of leap years also needed to be changed. In the Julian calendar, every 4th year was a leap year, but it was realized that this created too many leap years and caused errors. The Gregorian calendar therefore used a modified and more complicated formula than the Julian calendar to calculate whether a year should be a leap year.
One of the most interesting facts about the Gregorian calendar is that because of the problems caused by the Julian calendar, they had to skip 10 days when the new calendar was introduced in 1582 in order to make up for previous errors. The final day of the Julian calendar was October 4th 1582 and the following day (the first day of the new Gregorian calendar) was October 15th 1582. The cycle of weekdays was not affected, however.
Although Pope Gregory had no control over anywhere apart from the Vatican and the Papal States, a handful of Catholic countries, including Portugal, Spain and most of Italy adopted the calendar almost straight away.
Because of the Protestant Reformation, many other countries were late to adopt the new calendar. There was considerable resistance in some Protestant countries to it and some people even saw the calendar as an attempt to bring Catholicism back to Protestant countries.
Britain and British Empire (including what is now the eastern part of the United States), adopted the new calendar in 1752.
Some countries didn’t switch from the Julian calendar to the Gregorian until the 20th Century. These includeplaces such as: China, Turkey, Greece and Russia. Because so much extra time had elapsed since the original introduction by Gregory in 1582, these countries had to skip 13 days instead of 10 to make up for the Julian error.