Which calendar reform made January 1 the official start of the year in much of the Christian world?

The Gregorian calendar reform made January 1 the official start of the year in much of the Christian world.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct the Julian calendar’s accumulated drift against the solar year. The reform removed ten calendar dates and changed the leap-year rule, making century years leap years only when divisible by 400. This keeps the calendar more closely aligned with the seasons.

The reform retained January 1 as New Year’s Day, a date already used in the Roman and Julian traditions. Adoption was gradual: Catholic countries generally changed first, while Britain and its colonies adopted the Gregorian calendar in 1752 and Russia adopted it for civil use after the 1917 Revolution.

The Gregorian calendar is now the international civil standard, but some churches and communities still use the Julian calendar for religious observances. That is why Christmas and New Year dates can appear different in different traditions.