Janus (jāˈnəs) [key], in Roman religion, god of beginnings. He was one of the principal Roman gods, the custodian of the universe. The first hour of the day, the first day of the month, the first month of the year (which bears his name) were sacred to him. His chief function was as guardian deity of gates and doors. The gates of his temple in the Roman Forum were closed in time of peace and opened in time of war. Janus was usually represented with two bearded heads placed back to back so that he might look in two directions at the same time. His principal festival was celebrated on the first day of the year.