In many countries the New Year begins on January 1. However, this wasn’t always the case. In fact, for centuries, other dates marked the start of the calendar, including March 25 and December 25. So how did January 1 become New Year’s Day?
We can partly thank the Roman king Numa Pompilius. According to tradition, during his reign (c. 715–673 BCE) Numa revised the Roman republican calendar so that January replaced March as the first month. It was a fitting choice, since January was named after Janus, the Roman god of all beginnings; March celebrated Mars, the god of war. (Some sources claim that Numa also created the month of January.) However, there is evidence that January 1 was not made the official start of the Roman year until 153 BCE.
In 46 BCE Julius Caesar introduced more changes, though the Julian calendar, as it became known, retained January 1 as the year’s opening date. With the expansion of the Roman Empire, the use of the Julian calendar also spread. However, following the fall of Rome in the 5th century CE, many Christian countries altered the calendar so that it was more reflective of their religion, and March 25 (the Feast of the Annunciation) and December 25 (Christmas) became common New Year’s Days.
Sourced from https://www.britannica.com/story/why-does-the-new-year-start-on-january-1