Janus is the ancient Roman god associated with January, the month that begins the Gregorian calendar year.
Janus was linked with doors, gates, beginnings, transitions, and passages. Roman representations often show him with two faces, one looking forward and the other backward, making him a fitting symbol for the boundary between an ending and a beginning.
The Latin name for January is Ianuarius, derived from Janus. The month became the first month of the Roman calendar in 153 BCE for practical political and military reasons: newly elected consuls began their terms in January, allowing them to organize campaigns earlier in the year.
The Gregorian calendar retained January as the first month when it was introduced by Pope Gregory XIII in 1582. Not every historical calendar or culture began the year in January, and some New Year celebrations still follow lunar or religious calendars. Thus Janus explains the month’s name and symbolism, not a universal rule about when every society celebrates New Year.