The ancient Roman god associated with doorways and transitions, and the source of January's name, was Janus.
Janus was linked with beginnings, endings, gates, passages, and changes. Roman artists commonly represented him with two faces, allowing him to look forward and backward at the same time. That image later became a powerful symbol for New Year's reflection: looking back at the completed year while looking ahead to the next one.
January is named for Janus. In the Roman calendar reform associated with the second century BCE, January became the first month of the civil year, although Roman New Year practices and calendar arrangements changed over time. The month remained connected with renewal and official beginnings.
The Roman festival of Agonalia for Janus was observed on January 9, while the Kalends of January marked the opening of the month with religious and social customs. Modern New Year's Eve celebrations are not direct recreations of those rites, but the language and symbolism of new beginnings preserve Janus's influence.