New Year's Day, the day immediately following New Year's Eve, falls on January 1 in the Gregorian calendar. New Year's Eve is December 31, so the two dates mark the final evening and first day of the calendar year respectively.
January 1 became the first day of the year in the Roman calendar tradition and was retained by the Julian calendar introduced by Julius Caesar in 46 BC. The Gregorian calendar, adopted from 1582 onward by many countries, also kept January 1 as New Year's Day. Although the Gregorian calendar is internationally dominant, some cultures and religions celebrate additional new years on different dates.
The date can be locally complicated when people travel across time zones: one place may already be on January 1 while another remains on December 31. Lunar, lunisolar, and religious calendars also produce celebrations such as Lunar New Year, Rosh Hashanah, and Nowruz, which are separate from Gregorian New Year's Day.