The Roman god who gives January its name is Janus.
Janus was associated with beginnings, transitions, doorways, gates, and passages. His name is the source of Ianuarius, the Latin name that became “January” in English. Because New Year's Day begins a new year, Janus became a natural symbolic figure for the transition from what has passed to what is beginning.
Janus is commonly depicted with two faces, one looking forward and one looking backward. This image reflects his connection with thresholds and change, rather than describing a single New Year's Eve ceremony. The Roman calendar reforms that established January 1 as the year's opening helped strengthen the month's association with beginnings.
Roman New Year customs included religious observances and the exchange of good wishes or gifts. Modern celebrations are not direct copies of Roman rites, but the language and symbolism remain. Janus should not be confused with Saturn, whose festival Saturnalia was a separate Roman celebration held in December.