Janus gave January its name, linking the Roman month to New Year celebrations. In Roman religion, Janus was associated with beginnings, transitions, doorways, and passages.
The month of January is named for Janus because the god symbolically looked both backward and forward. That image suited the start of a new year: people could reflect on the year that had ended while anticipating the one ahead. Janus was commonly represented with two faces, although Roman representations and interpretations were more varied than the simplest modern summaries suggest.
January 1 became an important Roman civic date after calendar reforms, but New Year's customs changed across time and places. Julius Caesar's calendar reform in 46 BCE helped establish a 365-day calendar with January 1 as the year's beginning in Roman administration. The later Gregorian calendar retained January 1 as New Year's Day for most countries.