Which calendar reform made January 1 the official New Year's Day in much of Europe?

The Gregorian calendar reform made January 1 the official New Year's Day in much of Europe.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct accumulated drift in the Julian calendar. The reform removed ten calendar dates and changed the leap-year rule, bringing the calendar more closely into line with the astronomical seasons. It retained January 1 as the beginning of the civil year in countries adopting it.

Adoption was not simultaneous. Catholic countries such as Italy, Spain, Portugal, and parts of the Polish-Lithuanian Commonwealth adopted the reform in 1582, while many Protestant and Orthodox countries changed later. Britain and its colonies adopted it in 1752, and Russia adopted it for civil use after the 1917 Revolution.

Some cultures continue to celebrate other traditional New Year dates, including Lunar New Year and Nowruz. The Gregorian calendar therefore dominates international civil administration but does not erase regional calendars or religious observances.