Here's a fantastic article from BAR in regards to 25th December. In a nutshell:
1. There is no evidence linking 25th December to the birth or worship of Nimrod and/or Tammuz.
2. There is no evidence that Nimrod was worshiped in the ancient world by anybody anywhere.
3. Nimrod was not Baal.
4. Baal was not a sun-god, he was the storm-god (Iškur in Sumer, Addu in Akkad, Hadad in Canaan).
5. Nimrod had nothing to do with Babylon; he lived at least 1,000 years before Babylon was founded.
6. The birth of Christ was celebrated in Egypt prior to 200 a.d.
7. The 3rd century is not a period of history during which the early Church borrowed from pagans.
8. No Christian writer of the 4th century acknowledges a pagan origin for Christmas.
9. The idea that Christmas had pagan origins did not appear in Christian writings until the 12th century.
10. December 25th may have been chosen because it was nine months to the day after 25th March, the Feast of the Annunciation, which celebrated the conception of Jesus (but we know his real birth date is 11th September 3 b.c.).