Which calendar reform moved New Year's Day to January 1 in many Catholic countries in 1582?

The Gregorian calendar moved New Year's Day to January 1 in many Catholic countries in 1582.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct the gradual seasonal drift of the Julian calendar. The reform skipped 10 calendar dates and changed the leap-year rule, keeping the calendar more closely aligned with the solar year. January 1 remained the civil New Year's Day in the countries that adopted the reform.

Catholic states such as Spain, Portugal, and much of Italy adopted the calendar immediately, while Protestant and Eastern Orthodox countries generally adopted it later. Britain and its colonies changed in 1752, and Russia adopted the civil Gregorian calendar after the 1917 Revolution.

The Gregorian calendar is now the world's most widely used civil calendar, but some churches still use the Julian calendar for liturgical dates. Calendar adoption therefore occurred at different times rather than in one worldwide change.