January 1 Becomes New Year’s Day
The early Roman calendar consisted of 10 months and 304 days, with each new year beginning at the vernal equinox; according to tradition, it was created by Romulus, the founder of Rome, in the eighth century B.C. A later king, Numa Pompilius, is credited with adding the months of Januarius and Februarius. Over the centuries, the calendar fell out of sync with the sun, and in 46 B.C. the emperor Julius Caesar decided to solve the problem by consulting with the most prominent astronomers and mathematicians of his time. He introduced the Julian calendar, which closely resembles the more modern Gregorian calendar that most countries around the world use today.
As part of his reform, Caesar instituted January 1 as the first day of the year, partly to honor the month’s namesake: Janus, the Roman god of beginnings, whose two faces allowed him to look back into the past and forward into the future.
Excerpt copied from: http://www.history.com/topics/holidays/new-years