The ancient Roman god associated with January and beginnings is Janus.
Janus was associated with doors, gates, transitions, passages, and beginnings. Roman art commonly portrayed him with two faces, allowing him to look both backward and forward. That image made him especially suitable for the symbolic boundary between an old year and a new one.
The month of January takes its English name from the Latin Ianuarius, meaning the month of Janus. In the Roman calendar reform traditionally attributed to King Numa Pompilius, January became the first month, although calendar conventions changed at different points in Roman history.
Julius Caesar’s calendar reform, introduced in 46 BCE and taking effect in 45 BCE, helped establish January 1 as the start of the Roman civil year. The Gregorian calendar later refined the system and is now widely used internationally. Janus was not simply a god of New Year celebrations; his broader role covered many kinds of beginnings and transitions.