The Gregorian calendar was introduced by Pope Gregory XIII in 1582 and is now the civil calendar used for most New Year's dates.
The reform corrected a small but accumulating error in the Julian calendar's calculation of the solar year. Without adjustment, calendar dates were gradually drifting away from the seasons. The Gregorian system changed the leap-year rule so that century years are not leap years unless divisible by 400.
The first countries adopted the reform in 1582, but others changed at later dates. This means that historical documents can show different dates for the same event depending on which calendar a country used. The British Empire, for example, adopted the Gregorian calendar in 1752.
The calendar takes its name from Gregory XIII, not from the celebration itself. New Year's Day is January 1 in the Gregorian system, although cultures and religions around the world also observe other new-year dates based on lunar, lunisolar, or regional calendars.