The first day of the year in the ancient Roman calendar was the Kalends of January, when new consuls took office.
The Roman calendar divided months around three named points: the Kalends, the Nones, and the Ides. The Kalends always fell on the first day of a month, so the Kalends of January meant January 1. The Latin name was Kalendae Ianuariae.
January 1 became especially important after 153 BCE, when the Roman consulship began on that date rather than March 15. The change helped Roman commanders take office and begin military operations sooner in the campaigning season.
The Julian calendar reform of 46 BCE retained January 1 as the beginning of the civil year. Modern New Year’s Day ultimately inherits this Roman calendrical tradition, although later calendar reforms changed the calendar’s rules for leap years and dates.