The Gregorian calendar began its first official use in 1582 under Pope Gregory XIII.
The reform corrected a gradual drift in the older Julian calendar, which treated the solar year as slightly too long. Over centuries, that small error shifted calendar dates away from the seasons. The Gregorian system adjusted the calendar by removing ten dates and refined the leap-year rule.
The first adopting countries skipped from Thursday, October 4, 1582, to Friday, October 15. The change was not adopted everywhere at once. Catholic countries generally accepted it first, while Britain and its colonies adopted it in 1752 and Russia adopted it after the 1917 revolution.
Today the Gregorian calendar is the civil calendar used by most of the world, although other calendars remain important for religious and cultural purposes. New Year's Day falls on January 1 in the Gregorian system, but communities using other calendars may celebrate a different date.