In the Gregorian calendar, New Year's Day is January 1, immediately after New Year's Eve.
New Year's Eve is December 31, the final day of the calendar year, while January 1 begins the next year. This pairing is used in countries and communities that follow the Gregorian calendar for civil purposes, although many cultures also observe other new years.
The date became part of the Gregorian calendar introduced by Pope Gregory XIII in 1582 to correct accumulated drift in the older Julian calendar. Adoption was gradual, and different countries switched at different times. The Gregorian calendar is now the most widely used civil calendar internationally.
January 1 was not the only historical choice for the start of a year. Earlier Roman traditions used other dates, and religious or lunisolar calendars may place New Year's celebrations elsewhere. That is why January 1 should be understood as the Gregorian civil date, not every culture's universal new year.