Janus, the Roman god of beginnings and transitions, gives January its name and is symbolized by two opposite-facing faces.
The Latin month name Ianuarius was associated with Janus, whose two faces were understood to look toward the past and the future. That symbolism made the god especially relevant to the transition from one year to the next. Janus was also connected with doors, gates, passages, and changes of state.
January became the first month of the Roman calendar after calendar reforms that increasingly treated the year as beginning on January 1. Roman consuls began their terms on that date from 153 BCE, helping establish its civic importance. Earlier Roman calendars had used different arrangements and did not always begin the year in January.
The god’s double face is not a claim that Janus was literally a monster with two separate heads. Ancient Roman art commonly represented him with a single head showing two profiles. The modern New Year idea of looking back and forward echoes this older association with thresholds and change.