Janus is the Roman god of doorways, gates, beginnings and endings, and other such "ways in" and "ways out". As such, he is the first deity invoked in prayer and ritual. The month January, named for Him, is also sacred to Him.
The Agonalia, also listed as Agonium, is celebrated at four times during the year, 17 March, 21 May, 11 December, and 9 January. On these days, the Rex Sacrorum would enter the Forum, and sacrifice a ram to the god. The exact meaning of the ceremony is not known, and even in the time of Ovid the meaning was not known. Just the same, the meaning of the name of the ceremony is also not known, but some have speculated that it comes from a statement of the ritual assistant.
The Rex Sacrorum was the only official particularly dedicated to Janus. There were certainly others who would have been dedicated to Janus; these would have been temple sacerdotes, as well as other unofficial attendants.