Which calendar did Pope Gregory XIII introduce in 1582, helping establish January 1 as New Year's Day in much of Europe?

The calendar introduced by Pope Gregory XIII in 1582 was the Gregorian calendar.

The reform corrected the Julian calendar's gradual drift relative to the seasonal year. Under the Julian system, the calendar year was slightly too long, causing dates such as the spring equinox to move over the centuries. The Gregorian reform adjusted the leap-year rules and removed several calendar dates during the initial change.

The reform did not make every country switch immediately. Catholic states generally adopted it first, while many Protestant and Orthodox countries continued using the Julian calendar for decades or centuries. Britain and its American colonies adopted the Gregorian calendar in 1752.

January 1 had already been used as a year-opening date in parts of the Roman and Christian worlds; the Gregorian reform standardized the calendar rather than inventing the idea of New Year's Day. Some Eastern churches still use the Julian calendar for liturgical dates.