A pagan Christmas?

I've often heard that Christians adopted the pagan winter solstice festival as the time to celebrate the birth of Jesus, and many of the practices of the pagan festivals were incorporated into our Christmas traditions (like Christmas trees). That hasn't bothered me too much throughout life because what matters to me is that Christ came.

But Gene Vieth seems to believe that this "pagan Christmas" is a myth. According to this article, Christmas actually started first, and pagans later tried to adopt the holiday as their own. Veith even gives some reasoning as to why Dec. 25th was picked as Jesus' birthday.

I'm not sure how accurate this is, but it gets me thinking (and wishing I had more time to research it!).