Which 12-month calendar sets January 1 as the first day of the year in most countries?

The Gregorian calendar sets January 1 as the first day of the year in most countries.

Pope Gregory XIII introduced the calendar in 1582 to correct the accumulated seasonal drift of the Julian calendar. The reform adjusted the leap-year rule so that the calendar year would better match the solar year. It also moved the date of the spring equinox closer to its position used in calculating Easter.

The Gregorian system retained January 1 as New Year’s Day, a date already used in the Roman calendar and in several European traditions. Countries adopted the reform at different times, so the calendar did not become globally standard all at once.

The Gregorian calendar is a solar calendar with 12 months and a normal year of 365 days. Its leap-year rule adds February 29 in years divisible by four, except most century years unless they are divisible by 400.