Julius Caesar established January 1 as the start of the civil year in the Julian calendar.
The Julian calendar reform took effect in 45 BCE after Caesar worked with the Alexandrian astronomer Sosigenes. It reorganized the Roman calendar to keep the civil year more closely aligned with the solar year and introduced a regular leap-year system.
January had already been connected with the beginning of Roman official business before the reform, but the Julian calendar made January 1 the standardized start of the civil year. The month was associated with Janus, the Roman god of beginnings, transitions, and doorways.
The Julian calendar was later corrected by the Gregorian reform because its leap-year rule made the calendar drift slowly against the seasons. Most countries now use the Gregorian calendar for civil purposes, but January 1 remains the familiar New Year's Day in much of the world because of this Roman calendar tradition.