Which calendar determines January 1 as New Year's Day in most countries today?

The Gregorian calendar determines January 1 as New Year's Day in most countries today.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct a small but accumulating error in the older Julian calendar. The Julian system treated the year as slightly too long, causing calendar dates to drift gradually against the seasons. The Gregorian reform adjusted the leap-year rule and removed several dates from the calendar at the time of adoption.

The new system was adopted first by Catholic countries and later by many other states. Adoption was gradual, so different countries used the Julian and Gregorian calendars simultaneously for periods of history. Today, the Gregorian calendar is the civil calendar used internationally for most official and commercial purposes.

January 1 was already recognized as New Year's Day in the Roman calendar tradition, but not every culture or religious calendar begins its year on that date. Lunar, lunisolar, and religious calendars can place their New Year celebrations elsewhere, including at dates that change from year to year.