January 1 begins immediately after New Year's Eve in the Gregorian calendar.
New Year's Eve is December 31, the final date of the Gregorian calendar year. At midnight, the date changes to January 1, which is New Year's Day and the first day of the new calendar year. This sequence is the basis for the familiar countdown from one year to the next.
The Gregorian calendar was introduced by Pope Gregory XIII in 1582 as a reform of the Julian calendar, mainly to correct the calendar's relationship with the solar year. Countries adopted it at different times, and some cultures also observe New Year according to other calendars. Thus January 1 is the Gregorian New Year, not the only possible New Year date worldwide.