According to the Gregorian calendar, New Year's Day follows New Year's Eve on January 1.
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 institutions that follow the Gregorian calendar for civil purposes.
January 1 became associated with the start of the year in the Roman calendar under Julius Caesar's reform, which took effect in 45 BCE. The Gregorian calendar, introduced by Pope Gregory XIII in 1582, retained January 1 as New Year's Day while correcting the calendar's drift against the seasons.
Some cultures and religions observe additional New Year festivals on different dates, including Lunar New Year and Rosh Hashanah. Those celebrations do not change the Gregorian date of January 1.