From wikipedia:
In ancient Roman religion and mythology, Janus is the god of beginnings and transitions,[1] thence also of gates, doors, doorways, endings and time. Most often he is depicted as having two faces on his head, facing opposite directions: one face looks eastward and the other westward. Symbolically they look simultaneously into the future and the past, back at the last year and forward to the next.
Jano es citado en la novela de Albert Camus, La caída, donde simboliza la dualidad del personaje entre el pasado y el futuro.
The interpretation of Janus as the god of beginnings and transitions is grounded onto a third etymology indicated by Cicero, Ovid and Macrobius which explains the name as Latin deriving it from the verb ire ("to go").[7] It has been conjectured to be derived from the Indo-European root meaning transitional movement (cf. Sanskrit "yana-" or Avestan "yah-", likewise with Latin "i-" and Greek "ei-".).[8] Iānus would then be an action name expressing the idea of going, passing, formed on the root *yā- < *y-eð(2)- theme II of the root *ey- go from which eō, ειμι.[9]
Other modern scholars object to an Indo-European etymology either from Dianus or from root *yā
In one of his works G. Dumézil has postulated the existence of a structural difference of level between the IE gods of beginning and ending and the other gods who fall into a tripartite structure, reflecting the most ancient organization of society. So in IE religions there is an introducer god (as Vedic Vâyu and Roman Janus) and a god of ending, a nurturer goddess and a genie of fire (as Vedic Saraswati and Agni, Avestic Armaiti, Anâitâ and Roman Vesta) who show a sort of mutual solidarity: the concept of god of ending is defined in connexion to the human referential, i.e. the current situation of man in the universe, and not to endings as transitions, which are under the jurisdiction of the gods of beginning, owing to the ambivalent nature of the concept. Thus the god of beginning is not structurally reducible to a sovereign god, and the goddess of ending to any of the three categories onto which the goddesses are distributed. There is though a greater degree of fuzziness concerning the function and role of female goddesses, which may have formed a preexisting structure allowing the absorption of the local Mediterrenean mother goddesses, nurturers and protectresses .[41] As a consequence the position of the gods of beginning would not be the issue of a diachronic process of debasement undergone by a supreme uranic god, but would rather be a structural feature inherent to the theology of such gods. The fall of uranic primordial gods into the condition of deus otiosus is a well known phenomenon in the history of religions. Mircea Eliade gave a positive evaluation of Dumezil's views and of the comparative research results on Indoeuropean religions achieved in Tarpeia.[42] even though he himself in many of his works observed and discussed the phenomenon of the fall of uranic deities in numerous societies of ethnologic interest.[43] The figure of the IE initial god (Vâyu, Vayu, Mainyu, Janus) may open the sacrifice (Vâyu and Janus), preside over the start of the voyage of the soul after death (Iranic Vayu), "stand at the opening of the drama of the moral history of the world" (the Zoroastrian Mainyus). They may have a double moral connotation, perhaps due to the cosmic alternance of light and darkness, as is apparent in the case of Zoroastrianism.