The reason for the reform is that the average length of the year in the Julian calendar is too long. It treats each year as 365 days, 6 hours in length, whereas calculations show that the actual mean length of a year is 365 days, 5 hours, and 49 minutes. As a result, the date of the actual vernal equinox, over the course of 13 centuries, has slowly slipped to March 10, while the calculation of the date of Easter still follows the traditional date of March 21.
These calculations are verified by the observations of mathematician and astronomer Christopher Clavius, and the new calendar is instituted when Gregory decrees on February 24, 1582, that the day after Thursday, October 4, 1582 will not be Friday, October 5, but rather Friday, October 15, 1582. The new calendar duly replaces the Julian calendar and has since come into universal use. Because of Gregory’s involvement, the reformed Julian calendar comes to be known as the Gregorian calendar.
The switchover is bitterly opposed by much of the populace, who fear it is an attempt by landlords to cheat them out of a week and a half of rent. However, the Catholic countries of Spain, Portugal, Poland, and Italy comply almost immediately. France, some states of the Dutch Republic, and various Catholic states in Germany and Switzerland follow suit within a year or two, and Hungary follows in 1587.
More than a century passes before Protestant Europe accepts the new calendar. Denmark, the remaining states of the Dutch Republic, and the Protestant states of the Holy Roman Empire and Switzerland adopt the Gregorian calendar in 1700–1701. Ireland and Great Britain, along with its American colonies, reform in 1752, where Wednesday, September 2, 1752 is immediately followed by Thursday, September 14, 1752. They are joined by the last Protestant holdout, Sweden, on March 1, 1753.
The Gregorian calendar is not accepted in eastern Christendom for several hundred years, and then only as the civil calendar. The Gregorian Calendar is instituted in Russia by the Bolsheviks in 1917. Romania accepts it in 1919 and is followed by Turkey in 1923. The last Orthodox country to accept the calendar is Greece, also in 1923.