The Roman god of beginnings and transitions, and thereby of gates, doors, doorways, passages and endings, Janus is depicted as having two faces, looking both to the future and to the past. A Janus charm on a bracelet was depicted in the 2010 movie (The Tourist) with Johnny Depp and Angelina Jolie. The creation story of my hand-carved version of the Janus pendant is told within my blog, here.
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 had functions pertaining to birth and to journeys and exchange, and in his association with Portunus, a similar harbor and gateway god, he was concerned with traveling, trading and shipping.
Janus had no flamen or specialized priest (sacerdos) assigned to him, but the King of the Sacred Rites (rex sacrorum) himself carried out his ceremonies. Janus had a ubiquitous presence in religious ceremonies throughout the year, and was ritually invoked at the beginning of each one, regardless of the main deity honored on any particular occasion.