What is the name of the 12-month calendar used for New Year’s Day on January 1 in most countries?

The Gregorian calendar is the 12-month calendar used for New Year’s Day on January 1 in most countries. It was introduced by Pope Gregory XIII in 1582 as a correction to the older Julian calendar.

The reform adjusted the calendar’s leap-year rule so that its average year more closely matched the length of the solar year. Without that correction, calendar dates gradually drifted away from seasonal events such as the spring equinox.

Catholic countries adopted the Gregorian system first, while many Protestant and Orthodox countries adopted it later. This created a period when different countries observed dates according to different calendars.

The Gregorian calendar is now the civil calendar used internationally, although religious and cultural calendars remain important. Some Orthodox churches still use the Julian calendar for liturgical dates, which is why certain Christmas celebrations occur in January.