New Year’s Day was first celebrated on January 1 in 45 B.C.E. when Julius Caesar reformatted the Roman calendar. It has been the first day of the year in most countries since the 17th century (1752 in Britain and its colonies).
January is derived from the Latin Ianuarius, which itself is derived from the Latin word ianua, which means door, and Janus the Roman god of gates, doorways and beginnings.