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?