If you’d like to bone up on the facts and myths about the Christian version of the Christmas story, this (Jewish) website explains everything very nicely.
For example, “How Did Christmas Come to Be Celebrated on December 25?”
Roman pagans first introduced the holiday of Saturnalia, a week long period of lawlessness celebrated between December 17-25. During this period, Roman courts were closed, and Roman law dictated that no one could be punished for damaging property or injuring people during the weeklong celebration. The festival began when Roman authorities chose “an enemy of the Roman people” to represent the “Lord of Misrule.” Each Roman community selected a victim whom they forced to indulge in food and other physical pleasures throughout the week. At the festival’s conclusion, December 25th, Roman authorities believed they were destroying the forces of darkness by brutally murdering this innocent man or woman.
Well, isn’t that pleasant…
(Thanks to Marilin for the link!)