I asked this question to a holy roller guy I used to work with. He told me the date of 25 December was originally a pagan holiday, and the assignation of the Christmas holiday to this date was actually the result of a political compromise by a king of England. I have no reason to disbelieve him!
You should. Christmas was celebrated on December 25th hundreds of years before there was a King of England.
The 1st Recorded Celebration of Christmas
http://www.christianity.com/church/church-history/timeline/301-600/the-1st-recorded-celebration-of-christmas-11629658.html
There is some research that I saw on EWTN (although this research is not conclusive - yet) that uses past astronomical data (which can be determined using the precise movement of planets in conjunction with computers) in an attempt to date exactly when the magi would have noticed “we have seen his star in the East and are come to worship him).
It would seem that the king planet, Jupiter, entering Virgo the Virgin at 2-3 B.C. announced to them that the King planet was entering the constellation of Virgo the Virgin, in connection with Saturn (the planet then ascribed as having significance to the Jewish people) and thus a King was being born in Judea.
What he’s probably referring to in the Roman feast of “Sol Invictus” or the feast of the “unconquerable sun” which was celerated on December 21st or 22nd(the winter solstice). The Roman church did this a lot, providing christian alternatives to pagan festivals.