Which calendar reform made January 1 the official start of the year in much of Europe from 1582?

The Gregorian calendar made January 1 the official start of the year in much of Europe from 1582.

Pope Gregory XIII introduced the calendar reform in 1582 to correct the gradual drift between the calendar date and the astronomical seasons. The reform removed ten calendar days and changed the rules for leap years, producing the system used by most countries today.

January 1 had already been used as the Roman civil New Year in earlier periods, and the Julian calendar also placed the year's start on January 1. The Gregorian reform therefore corrected the calendar's structure rather than inventing the January date from nothing.

Adoption was not simultaneous. Catholic countries generally adopted the reform first, while Britain and its colonies switched in 1752 and Russia adopted it for civil use after the 1917 Revolution. Some religious calendars still use other systems.