Ok, I think we've got enough
information about the December
25th date. What about the January 6th
date?
"After the triumph of
Constantine, the church at Rome assigned December 25 as
the date for the celebration of the feast, possibly
about A.D. 320 or 353. By the end of the fourth century the whole
Christian world was celebrating Christmas on that day,
with the exception of the Eastern churches, where it was
celebrated on January 6. The choice of December 25
was probably influenced by the fact that on this day the
Romans celebrated the Mithraic feast of the Sun-god (natalis
solis invicti), and that the Saturnalia also came at
this time." - Colliers Encyclopedia