Janus, the Roman god of beginnings and transitions, gives its name to January, the month beginning the New Year in the Gregorian calendar.
The Romans associated Janus with doorways, gates, passages, and changes from one state to another. He was commonly represented with two faces, one looking forward and one looking backward, making him an especially fitting symbol for the transition between years.
The Latin month name Ianuarius became January in English. January is the first month in the modern Gregorian calendar, which was introduced by Pope Gregory XIII in 1582 and is now the most widely used civil calendar.
Janus is not the same deity as Saturn, whose festival Saturnalia was celebrated in December, or Mars, whose name survives in March. The association with New Year concerns the month’s name and symbolism, not a claim that Janus invented the modern holiday.