Christmas celebrates the birth of Jesus Christ, but the academic consensus is that there is no definite evidence for the month in which Jesus was born.
The tradition of celebrating his birth later in the year started in the fourth century. The marking of the mid-winter solstice in the Northern Hemisphere, around 23 December, with its powerful connections to the emergence from winter and the celebration of light, warmth, fecundity and positive powers, meant that many religions and cultures had celebrations then.
In the Roman world, the major festival was Saturnalia, held in honour of Saturn, the father of the major gods of the Greeks and Romans.
The celebration of the birthday of Mithras, the sun god of the Persians whose mystery religion was popular among soldiers in the Roman army, was also celebrated on 25 December.
Mithras was often perceived as a rival to Jesus Christ, and Christians took advantage of the popularity of the 25 December date to celebrate the birth of their own influential religious figure.
A Mithraic relief found near Rome in 1926.
The popularity of decorating a pine tree as part of Christmas also has its origins in non-Christian traditions from the northern parts of Europe, where the festival of Yule was celebrated at the mid-winter solstice.
The traditional Christmas tree is an evergreen, whose green branches defy the chill winter and point to the ultimate victory of the sun.
Our big-bellied, bearded giver of gifts has his origins in several equally generous figures. Saint Nicholas was a Greek bishop in the fourth century who provided dowries to poor girls to prevent them turning to prostitution.
The figure of Kris Kringle is derived from the German Christ Kindl (Christ child) who was the gift-giver in the German-speaking world. His tradition was brought to the United States by Pennsylvania Dutch people, among whom he is still popular.
Boxing Day grew out of the world of work. The tradition, which some claim may extend back to the late Roman period, but which is probably medieval, is that employers would give their apprentices boxes containing gifts. Masters would do the same for their servants.
Have you ever wondered why we call it "Easter"? Professor Carole Cusack, from the Department of Studies in Religion at the University of Sydney, reflects on the origins of some of the more familiar elements of the Easter season.