The Gregorian calendar reform began in 1582 in the first countries that adopted it.
Pope Gregory XIII introduced the calendar through the papal bull Inter gravissimas, issued in February 1582. Catholic countries including Spain, Portugal, and much of Italy implemented the change later that year. In those places, Thursday, October 4, 1582, was followed by Friday, October 15, removing ten calendar dates to realign the calendar with the solar year.
The reform corrected drift in the older Julian calendar, whose estimate of the length of the year was slightly too long. The Gregorian system adjusted the leap-year rule: century years are not leap years unless divisible by 400. That is why 2000 was a leap year, while 1900 was not.
Adoption was gradual rather than worldwide on one date. Britain and its colonies adopted the Gregorian calendar in 1752, while some Eastern European countries adopted it much later. Modern New Year's dates generally follow the Gregorian calendar.