Which calendar system makes January 1 the official New Year's Day in most countries?

The Gregorian calendar makes January 1 the official New Year's Day in most countries.

Pope Gregory XIII introduced the calendar in 1582 to correct the gradual seasonal drift of the Julian calendar. Its improved leap-year rule more closely matches the length of the tropical year: century years are not leap years unless divisible by 400. This change helped keep calendar dates aligned with the seasons over long periods.

Adoption was gradual rather than worldwide on one date. Catholic countries generally adopted the reform first, while other states changed later. Some religious communities still use other calendars for ceremonial or liturgical purposes, but the Gregorian calendar is the dominant civil calendar and gives January 1 its widespread official status.