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

The calendar reform that made January 1 the official start of the year in much of the Western world from 1582 was the Gregorian calendar.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct the Julian calendar’s accumulated drift relative to the solar year. The reform removed 10 calendar dates from October 1582 and changed the leap-year rule: century years are not leap years unless divisible by 400.

The Gregorian system retained January 1 as New Year’s Day, a date with ancient Roman and Christian administrative history. Adoption was not simultaneous. Catholic countries generally adopted it first, while Britain and its colonies changed in 1752 and Russia adopted it for civil use after the 1917 Revolution.

The Julian calendar remains used by some churches for liturgical purposes, so Christmas and New Year dates can differ between communities. The Gregorian calendar is now the internationally dominant civil calendar, but other calendar systems continue to be used for religious and cultural observances.