In the mythology of ancient Rome, Janus was the god of new beginnings. He was associated with doors and gates, and the first steps of a journey. The month of January -- of course, falling at the beginning of the new year -- is named in his honor. He is often invoked together with Jupiter, and is considered a high-ranking god.
In many portrayals, Janus is depicted as having two faces, looking in opposite directions. In one legend, Saturn bestows upon him the ability to see both the past and the future. In the early days of Rome, city founder Romulus and his men kidnapped the women of Sabine, and the men of Sabine attacked Rome in retaliation. The daughter of a city guard betrayed her fellow Romans and allowed the Sabines into the city. When they attempted to climb the Capitolian Hill, Janus made a hot spring erupt, forcing the Sabines to retreat.
In the city of Rome, a temple known as the Ianus geminus was erected in Janus' honor and consecrated in 260 b.c.e. after the Battle of Mylae. During periods of war, the gates were left open and sacrifices were held inside, along with auguries to predict the results of military actions. It is said that the gates of the temple were only closed in times of peace, which didn't happen very often for the Romans. In fact, it was later claimed by Christian clerics that the gates of the Ianus geminus first closed at the moment that Jesus was born.
Because of his ability to see both back and forward, Janus is associated with powers of prophecy, in addition to gates and doors. He is sometimes connected with the sun and moon, in his aspect as a dual-headed god.