Which calendar did Pope Gregory XIII introduce in 1582, fixing January 1 as New Year's Day in many countries?

The Gregorian calendar is the calendar Pope Gregory XIII introduced in 1582, helping establish January 1 as New Year's Day in many countries.

Pope Gregory XIII promulgated the Gregorian calendar in 1582 to correct a small but accumulating error in the Julian calendar's calculation of the solar year. The reform removed ten calendar days from October 1582 and changed the leap-year rule. Years divisible by 100 would no longer be leap years unless also divisible by 400.

January 1 had already served as the beginning of the civil year in parts of Europe and in ancient Rome, so the Gregorian reform did not invent the date. Instead, the calendar gradually spread through Catholic and later Protestant and Orthodox countries. Britain and its colonies adopted it in 1752, while Russia adopted it for civil use after the 1917 revolution.

Not every culture uses January 1 as its only or primary New Year date. Religious and traditional calendars continue to mark other New Year celebrations, including Lunar New Year and Rosh Hashanah.