Do Mithras, Osiris, Adonis, and Dionysus share a December 25 birthday?

Question

In The Da Vinci Code Dan Brown claims that Mithras, Osiris, Adonis, and Dionysus were born on December 25:

Teabing groaned. 'Don’t get a symbologist started on Christian icons. Nothing in Christianity is original. The pre-Christian God Mithras – called the Son of God and the Light of the World – was born on December 25, died, was buried in a rock tomb, and then resurrected in three days. By the way, December 25 is also the birthday of Osiris, Adonis, and Dionysus. The newborn Krishna was presented with gold, frankincense, and myrrh. Even Christianity’s weekly holy day was stolen from the pagans.'

I realize the book isn't known to be faithful to ancient myths, and the author probably was very liberal in interpreting his sources in order to create dramatic connections that fit the plot. However, I'm assuming that at the very least there are ancient sources pointing to a December birth for each of the aforementioned deities. Or are there?

Where did Dan Brown get the idea that Mithras, Osiris, Adonis, and Dionysus share a December 25 birthday? Is there any basis for the claim in ancient myths?

Answer 1

Since historians started noted how Christianity attempted to displace paganism, assertions such as Dan Brown's are a commonplace. Older cultures did not have a precise calendar, so Dec. 25th is approximately the winter solstice, a period of a few days when the duration of daylight is minimal and which gets more perceptible in the northern latitudes. The "Day" or the "Sun" starts "growing" after the solstice and for sedentary peoples the fact has an obvious importance, which apparently has been magnified by later commentators. Macrobius' (5th c.) interest in the traditional Roman Saturnalia is an influent source mentioning Dionysus and Horus. Mithra, whose genealogy is rather complicated, in imperial times was assimilated to Sol Invictus (the unconquered sun) and so he came to be celebrated at the solstice. Adonis fits well the schema of a rebirth but it is not clear when it was decided that he should be born on Dec. 25th. The so called Cambridge Ritualists have a significant role for the popular connection of the myths and calendar.

Answer 2

I'm glad you asked this, because this is something I've wondered about for awhile. I think the other answers cover Mithra and Adonis pretty well, so here's what I have been able to find out about Dionysus and Osiris.
The rural Dionysia (a Greek festival) was held to celebrate the wine harvest, but it actually took place in the month of Poseidon, which "straddled the winter solstice" (Wikipedia). It seems to have been a lot like the Roman Saturnalia.
The Wepet-Renpet Festival, which celebrated the death and rebirth (well, sort of) of Osiris was the New Year festival of Ancient Egypt, but unfortunately their new year was in July, when the Nile flooded.
Hope this helps.