Janus, the Roman god of beginnings and transitions, inspired the name of January.
Janus was traditionally depicted with two faces, one looking forward and the other backward. That imagery suited the first month of the Roman year because January looked toward the future while also reflecting on the year that had ended. The Latin name Ianuarius developed from the god's name.
January was not always the first month of the Roman calendar. Early Roman traditions placed March at the beginning, but January and February were later added or repositioned. Roman calendar reforms eventually established January 1 as the start of the civil year, a convention inherited by the Julian calendar.
The modern Gregorian calendar also begins on January 1, although countries adopted that calendar at different times. January's name therefore preserves a Roman religious association even in societies that no longer follow the ancient Roman calendar.