What calendar, introduced in 1582, is used to define January 1 as New Year's Day in most countries?

The Gregorian calendar, introduced in 1582, is used to define January 1 as New Year's Day in most countries.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct the accumulated drift between the calendar date and the astronomical seasons. It adjusted the leap-year rule used by the Julian calendar: century years are not leap years unless divisible by 400. This makes the average calendar year more closely match the tropical year.

The reform was adopted first by Catholic countries, including Italy, Spain, Portugal, and parts of the Polish–Lithuanian Commonwealth. Protestant and Orthodox countries adopted it later, and some communities still use other calendars for religious or cultural purposes. Britain and its colonies adopted it in 1752.

January 1 was already a New Year's date in parts of the Roman and Christian world before the reform. The Gregorian calendar standardized the civil date internationally over centuries; it did not invent every New Year's custom. Lunar and lunisolar calendars still determine many traditional New Year celebrations.