Pope Gregory XIII's 1582 calendar reform helped restore January 1 as the beginning of the year in much of Europe.
The Gregorian calendar was introduced to correct the Julian calendar's accumulated drift from the solar year. Under the Julian system, the date of the spring equinox had gradually moved earlier, affecting the calculation of Easter. The reform removed ten calendar dates in 1582 and adjusted the leap-year rule.
January 1 had been an important Roman administrative New Year date and was already used in many places, but medieval Europe did not have a single universal New Year's date. Some regions began the year on March 25, Christmas Day, or another local date. The Gregorian reform standardized January 1 for countries that adopted it.
Adoption was gradual rather than simultaneous. Catholic countries generally accepted the new calendar first, while Protestant and Orthodox countries adopted it later. The Gregorian calendar is now the civil calendar used internationally, although some churches retain other calendars for religious observances.