Who decided that 1/1 should be New Years day? When did this happen? Could it ever change?
Hm.
Gotta start somewhere.
Basically because the Romans happened to pick that day and our calendar is more closely associated with the Roman calendar (specifically, the Julian calendar ordered by Julius Caesar), than with any other culture’s.
Why did the Romans pick that date? Probably because they calculated the year according to the sun. Their great festival of the death and rebirth of the sun, the Saturnalia, coincided with the end of December and at the end of the festival, the new month that began was January. Being tidy folks, they started the new year at the beginning of the month.
A casual glance at various histories will cause confusion, because after Rome fell, various medieval societies began using March 25 (roughly corresponding to the beginning of the year for the Zodiac) and any number of other dates (including Christmas and various dates in September). However, there was always a contingent of scholars who held out for the traditional Roman beginning of the year, and January 1 eventually beat out the opposition.