The Gregorian calendar is the calendar system introduced by Pope Gregory XIII in 1582 and used for January 1 New Year’s Day in much of the world.
It was created as a reform of the Julian calendar, whose small error in the length of the year had gradually shifted the calendar away from the seasons. The Gregorian reform adjusted the leap-year rule and removed several calendar dates when it was first adopted in Catholic countries.
The new system retained January 1 as the start of the civil year in places that followed that convention. Adoption was not immediate worldwide: countries and institutions switched at different times, and some Eastern Christian communities continue to use other calendars for particular religious observances.
The Gregorian calendar is therefore not the same as a universal religious calendar. It is the dominant civil calendar internationally, while January 1 New Year celebrations can coexist with lunar, religious, or regional new-year traditions.