The Gregorian calendar officially began in the papal states in 1582.
Pope Gregory XIII introduced the calendar through a papal decree called Inter gravissimas. The reform corrected the gradual drift between the calendar date and the astronomical seasons that had accumulated under the Julian calendar. In the first countries adopting it, Thursday, October 4, 1582, was followed by Friday, October 15, removing ten calendar dates.
The reform also changed the leap-year rule: century years are not leap years unless divisible by 400. That is why 1600 and 2000 were leap years, while 1700, 1800, and 1900 were not.
Adoption was gradual rather than worldwide. Catholic countries adopted the calendar first, while Britain and its colonies switched in 1752 and Russia adopted it for civil purposes after the 1917 Revolution. The modern January 1 New Year is therefore linked to a calendar system that spread over centuries.