Janus was a two-faced god of Roman mythology who kept the gate of Heaven. He was the god of beginnings and ends, and so of gates, doors, doorways and passages. He is usually depicted as having two faces, since he looks to the future and to the past. The Romans named the month of January (Ianuarius) in his honor. In January we look back on the last year and forward to the next.
Janus presided over the beginning and ending of conflict, and hence war and peace. The doors of his temple were open in time of war, and closed to mark the peace. As a god of transitions, he was remembered at births, and journeys: he was concerned with travelling, trading and shipping. It was also said that his face was split in two since a door can let you in or out.
An unusual feature of the mythology is that the ancient Greeks had no version of Janus.