In times long gone—before the story-telling of Near Eastern desert tribes who succeeded in devising an unlikely monotheistic faith for their own patriarchal purposes—country peoples worldwide from the Neolithic onwards had celebrated the winter and summer solstices as the prime celestial events of significance to agriculturists in need of a simple calendar.
21st December is the year’s shortest day. For some days at about this time in the northern-hemisphere winter, the day length between sunrise and sunset hardly changes. Only a few people (like, let us say, a tribe’s official sun-watcher, and inquisitive archaeologists like me) who watch the point of sunrise daily at the horizon would notice the tiny difference between any of the days from 19th to 25th December.
There are many web sites discussing the detail of “the origin of christmas”, among them this christian one: http://www.hope-of-israel.org/christ~3.htm
Quoting Barbara Walker (1983. Encyclopedia,
p.166, Harper Row) regarding the Christian search for a day to celebrate, “some favored the popular date of the Koreion, when (she was citing Joseph Campbell, Mythic Image
p.34) the [supposedly] divine virgin gave birth to the new Aeon in Alexandria.” This different date came to be called Twelfth Night or Epiphany, and is a date celebrated by Armenian churches of the Greek Orthodox. On the other hand the fourth-century Roman church “favored the Mithraic winter-solstice festival called Dies Natalis Solis Invictus,
Birthday of the Unconquerable Sun. Blended with the Greek sun-festival of the Helia by the emperor Aurelian, this December 25 nativity also honored such gods as Attis, Dionysus, Osiris, Syrian Baal, and other versions of the solar Son of Man.”
Thus it came to pass that christian bishops at one of their 4th century pan-European frolics—seeing that no-one knew when their hero was born—deliberately picked the cross-cultural day that everybody already loved because of its importance to pagan farming communities and widely loved as a god’s festive occasion.
That is how celebration of the prehistoric winter solstice came to be usurped via the Mithraic cult by a desperate church, and the day of choice got shifted from the shortest day to 25 December.
As non-believers in religious trumpery, there is a good case for atheists being logically better off in recognising 25 December as “Newtonmass” or “Newtonday”, for which please refer to the Nexus discussion note that follows this one.