In the Western world, we celebrate the New Year on January 1st because of Julius Caesar, whose eponymous Julian calendar began in Mensis Ianuarius (or Januarius) 45B.C.. The month of January is named after the Roman god Janus, who had two faces, one facing the future and the other the past. He was the god of beginnings, endings, and transitions, or, more prosaically, doors and passageways.
Over time, the Julian calendar would prove to be inaccurate, so starting in the sixteen century, the reformed Gregorian calendar began to be adopted.
As A.F. Pollard explains it, there have been other contenders for New Year’s Day since Rome called the shots. In ancient times, March 1st marked the beginning of the year; in the Middle Ages, March 25th—Annunciation in the Christian ecclesiastical calendar—was a strong contender. (This was also supposed to be the anniversary of the day the world was created.)
It’s perhaps coincidental that March was also when U.S. Presidents were originally inaugurated. This was because March 4th—the anniversary of the Constitution replacing the Articles of Confederation as the law of the land—was celebrated as the beginning of a new political season. The 20th Amendment changed inauguration to January 21th in 1933 because four months between election and inauguration were seen as being entirely too long.
What may not be so coincidental is that in the Northern hemisphere, March is the beginning of spring, a very fitting time to begin a new year of life. The winter solstice’s proximity to both Christmas and New Year are also not randomly associated; days start getting longer after the solstice, another great way to begin a new year.
Meanwhile, the Christian calendar, however secularized over time, is not the world’s only way of keeping time. The Jewish New Year, Rosh Hashanah, has its beginnings in the Middle Eastern agricultural cycle. Like the Islamic calendar, the Jewish is a lunar one. The two-day Jewish New Year in 2016 will begin on October 2nd. The Muslim New Year, Raʼs as-Sanah al-Hijrīyah, begins Oct 3rd, 2016. In India, meanwhile, there are several types of new year’s days that reflect the rich cultural mixture of regions and peoples there.
In theory, one could spend a good part of the year celebrating new year’s.