Janus gives January its name and is linked to New Year's beginnings.
Janus is the ancient Roman god associated with beginnings, transitions, doorways, passages, and endings. January, called Ianuarius in Latin, was named for him. His connection with looking backward and forward made him especially suitable to the symbolic transition from one year to the next.
Roman representations commonly show Janus with two faces, one facing toward the past and the other toward the future. This imagery is a later shorthand for his role as a guardian of thresholds rather than evidence that he was a god of New Year’s Eve in the modern sense.
The Roman calendar changed over time, and January was not always treated in exactly the same way in the earliest Roman system. Julius Caesar’s calendar reform helped establish January 1 as the beginning of the Roman civil year, a convention that strongly influenced later European calendars.