January 1 marks New Year's Day, immediately following New Year's Eve in the Gregorian calendar.
New Year's Eve is December 31, while New Year's Day begins at midnight and continues through January 1. The two dates are often grouped together as the New Year holiday, but they have different names and traditional activities.
January 1 became the opening day of the Roman republican calendar in 153 BCE, when Roman consuls began taking office on that date. Julius Caesar's calendar reform later helped standardize the date, and the Gregorian calendar retained it.
Not every culture begins its traditional or religious new year on January 1. Lunar, lunisolar, and religious calendars use different dates, which is why New Year celebrations occur at several times around the world.