The Roman god depicted with two faces who gives January its name is Janus.
Janus was associated with beginnings, transitions, doorways, passages, and endings. His two faces symbolically looked in opposite directions, making him a natural figure for the change from one year to the next. The Latin name for January, Ianuarius, derives from his name.
The Roman calendar reform associated with Numa Pompilius placed January at the beginning of the year, although Roman calendar arrangements changed over time. Julius Caesar’s calendar reform in 46 BCE helped establish the Julian calendar, in which January remained the first month.
Janus is often mistakenly described simply as the Roman equivalent of a Greek god. He has no exact Greek counterpart and is generally regarded as a distinct Roman deity. His connection with New Year’s celebrations comes primarily through the month’s name and his broader role as a guardian of beginnings, rather than from a single Roman midnight ceremony identical to modern celebrations.