Ianuarius
Januarius was initially the eleventh month in the year of office of the Roman calendar as well as precursors of January. The month originally had 29 days. The attribution has the Roman god Janus as a basis, the Roman god of beginnings and endings, the inputs and outputs of the doors and gates. His name belongs to the same word family as ianua, the Latin word for door.
In the year 153 BC, the annual commencement of the year was moved to January 1, so the month Januarius moved to the first position in the year of office. After the calendar reform of Gaius Julius Caesar was in the Julian calendar from the year 45 BC by the insertion of two additional days for an extension to 31 days.