"The Roman ruler Julius Caesar established January 1 as New Year's Day in 46 BC. The Romans dedicated this day to Janus, the god of gates, doors, and beginnings. The month of January was named after Janus, who had two faces - one looking forward and the other looking backward." (World Book Encyclopedia)
So Caesar felt that the month named after this god (“January”) would be the appropriate “door” to the year.
Therefore, New Year's celebrations are founded on pagan traditions and as Christianity spread, many pagan holidays were incorporated into the Christian calendar.
Praise be to Allah for the purity of Islam which is void of all paganistic rituals and festivals.