Pope Gregory XIII introduced the Gregorian calendar reform that retained January 1 as New Year’s Day in the 16th century.
The calendar was promulgated in 1582 through the papal bull Inter gravissimas. Its main purpose was to correct the accumulated drift between the calendar date and the astronomical seasons, especially the date used to calculate Easter. The reform adjusted the leap-year rule: century years are not leap years unless divisible by 400.
The Gregorian calendar preserved January 1 as the start of the civil year in the systems that adopted it. Adoption was not immediate worldwide. Catholic countries generally changed first, while many Protestant, Orthodox and other states adopted the calendar later. Britain and its colonies, for example, adopted it in 1752.
January 1 had already been used as a New Year date in earlier Roman and European traditions, so Gregory XIII did not invent the concept of New Year’s Day. His reform standardized the calendar used by much of the modern world.