Why do we celebrate new year day on January 1st?

Did you think about this? Why do we celebrate new year day on January 1st? Why not on December 1st? Strange question huh? Let abstract my reading from this Wikipedia page. The Romans dedicated New Year's Day to Janus, the god of gates, doors, and beginnings for whom the first month of the year (January) is also named. After Julius Caesar reformed the calendar in 46 BC and was subsequently murdered, the Roman Senate voted to deify him on the 1st January 42 BC in honour of his life and his institution of the new rationalized calendar.

Source: New Year - Wikipedia

This is about history, let us all welcome the new year with full of hopes and prosperity! I wish you all a great year ahead!