Pope Gregory XIII introduced the Gregorian calendar, which retained January 1 as New Year's Day in many countries.
The calendar was promulgated in 1582 through the papal bull Inter gravissimas. Its principal 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 and skipped days during the initial changeover.
The Gregorian calendar was adopted first by Catholic countries, while many Protestant, Orthodox, and other regions adopted it later. This means historical documents can show different dates for the same event during periods when calendars coexisted.
January 1 had already served as New Year's Day in the Roman calendar and in the Julian calendar used before the reform. Gregory XIII therefore did not invent the January 1 tradition; he sponsored the calendar reform that became the modern civil standard in much of the world.