The Roman god Janus gives January its name in New Year's calendar history.
Janus was associated with beginnings, transitions, doorways, and passages. Roman artists often depicted him with two faces, one looking forward and one looking backward. That imagery made the god an apt symbol for the boundary between the old year and the new one.
The Latin name for January was Ianuarius, derived from Janus. January became the first month of the Roman calendar after calendar reforms associated with the Roman Republic, although the historical details of early Roman calendars are complex and changed over time.
The modern Gregorian calendar also begins on January 1. New Year's Day was not always observed uniformly on that date across Europe; different regions historically used dates such as March 25 or Easter as the start of the legal or religious year. The January date eventually became widely standardized.