The Gregorian calendar was introduced by Pope Gregory XIII and now determines January 1 for most international New Year's celebrations.
Pope Gregory XIII promulgated the calendar in 1582 through the papal bull Inter gravissimas. It corrected the accumulated drift between the calendar date and the astronomical seasons that had developed in the Julian calendar. The reform removed several dates from the first year of adoption and changed the leap-year rule.
Under the Gregorian system, century years are not leap years unless divisible by 400. Thus 2000 was a leap year, while 1900 was not. This rule keeps the calendar closely aligned with the seasonal year without making every year exactly the same length.
Adoption was gradual rather than worldwide on one date. Catholic countries generally adopted it first, while Britain and its colonies switched in 1752 and Russia adopted it for civil use after the 1917 Revolution. Some religious communities still use other calendars.