Which calendar reform made January 1 the official start of the year in many European countries?

The Gregorian calendar made January 1 the official start of the year in many European countries.

Introduced in 1582 by Pope Gregory XIII, the Gregorian calendar primarily corrected the Julian calendar’s drift relative to the solar year. The reform changed the leap-year rule and removed several calendar dates to realign the calendar with the seasons. January 1 remained the beginning of the civil year in countries adopting the reform.

Adoption was not simultaneous. Catholic countries generally accepted the reform first, while Protestant, Orthodox, and other regions adopted it later or retained different calendars for longer periods. Britain and its colonies adopted the Gregorian calendar in 1752, for example.

The calendar is now the international civil standard, although some churches and communities use other calendars for religious observances. New Year’s Day on January 1 should therefore be distinguished from local religious or traditional New Year celebrations that fall on different dates.