The month January is named after Janus, the Roman god associated with doorways, transitions, and beginnings.
Janus is traditionally depicted with two faces, one looking forward and one looking backward. That imagery made him an especially fitting symbol for the transition from one year to the next. His name is connected with the Latin word for door, and January functions metaphorically as a doorway into a new year.
The Roman calendar’s arrangement changed over time, but January 1 became an important date for political and civic business in the Roman Republic. The Julian calendar reform of 46 BCE helped regularize the calendar, and January remained the first month in the later Gregorian calendar used by much of the world.
Janus is not the god of the New Year in the same simple sense as a modern holiday mascot. His broader sphere was beginnings, thresholds, entrances, and endings. The connection with New Year’s Eve comes from the calendar month and its symbolism of looking backward and forward.