Civilizations around the world have been celebrating the start of each new year for at least four millennia. Today, most New Year’s festivities begin on December 31 (New Year’s Eve), the last day of the Gregorian calendar, and continue into the early hours of January 1 (New Year’s Day). Common traditions include attending parties, eating special New Year’s foods, making resolutions for the new year and watching fireworks displays.

https://www.history.com/topics/holidays/new-years

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.

https://www.history.com/topics/holidays/new-years