The Roman god who gives January its name and is associated with beginnings and transitions is Janus.
Janus was a Roman deity connected with doors, gates, passages, beginnings, endings, and transitions. He is commonly depicted with two faces, one looking forward and one looking backward. That image made him an enduring symbol for the passage from an old year into a new one.
The month January derives its name from Janus. In the Roman calendar, January became the first month in the second century BCE, although calendar arrangements changed over Roman history. The month’s association with Janus helped connect it with opening a new annual cycle.
Janus was not simply a god of New Year's celebrations in the modern sense. Romans also invoked him at the start of journeys, undertakings, and important transitions. The Latin name Ianuarius became the source of names for January in many European languages. The two-faced image is sometimes misunderstood as deceitful, but in Roman symbolism it primarily expressed Janus's ability to look in two directions at once.