What calendar date follows December 31 in the Gregorian calendar?

January 1 follows December 31 in the Gregorian calendar.

That sequence makes January 1 the first day of the civil year for countries and institutions using the Gregorian calendar. December 31 is therefore New Year's Eve, while the following day is New Year's Day. The two dates are connected but are not interchangeable.

The Gregorian calendar was introduced by Pope Gregory XIII in 1582 to correct accumulated drift in the earlier Julian calendar. Adoption happened at different times in different countries, and some Eastern Christian traditions continue to use other calendars for particular religious observances. In the modern international civil system, however, the transition from December 31 to January 1 is the standard New Year boundary.