Janus is the god of beginnings and transitions, thence also of gates, doors, passages, endings and time. 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.
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 travelling, 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.
The interpretation of Janus as the god of beginnings and transitions is based on a third etymology indicated by Cicero, Ovid and Macrobius, which explains the name as Latin, deriving it from the verb ire (“to go”).
Another etymology proposed by Nigidius Figulus is related by Macrobius: Ianus would be Apollo and Diana Iana, by the addition of a D for the sake of euphony. This explanation has been accepted by A. B. Cook and J. G. Frazer. It supports all the assimilations of Janus to the bright sky, the sun and the moon. It supposes a former *Dianus, formed on *dia- < *dy-eð2 from Indo-European root *dey- shine represented in Latin by dies day, Diovis and Iuppiter. However the form Dianus postulated by Nigidius is not attested.
Janus frequently symbolized change and transitions such as the progress of future to past, from one condition to another, from one vision to another, and young people’s growth to adulthood. He represented time, because he could see into the past with one face and into the future with the other. Hence, Janus was worshipped at the beginnings of the harvest and planting times, as well as at marriages, deaths and other beginnings. He represented the middle ground between barbarism and civilization, rural and urban space, youth and adulthood. Having jurisdiction over beginnings Janus had an intrinsic association with omens and auspices.
In Act I Scene 2 of Shakespeare’s Othello, Iago invokes the name of Janus after the failure of his premiere plot to undo the titular character. As the story’s primary agent of change, it’s fitting that Iago align himself with Janus. His schemes prompt the beginning of each of the main characters’ ends: in his absence, Othello and Desdemona would likely have remained married and Cassio would have remained in his respected position of power. Iago guides (if not forces) the story through inception, climax, and finale. Furthermore, Janus’ common two-faced depiction is the perfect visual metaphor for Iago’s character. Othello’s characters believe him to have only the best of intentions, even going as far as to call him “honest Iago,” completely unaware that he spends every unwatched second plotting their undoing. He appears selfless and compassionate but, in truth, is power-hungry, amoral, and without regard for the well-being of others.