The incorporation of paganism into Christianity was a tactic begun during the Dark Ages to allow the pagans to swallow Christianity more readily. Allow me to explain. In essence, most pagan Gods are strong, fierce, manly, muscled, sex-loving gods and the Goddesses are beautiful, seductive, fertile, and may or may not be the nagging type (like Hera) (please correct me if I'm wrong). Then comes in Christianity trying to get these people to embrace a God that is weak, was tortured to death, and died willingly without a fight. This was adverse to their beliefs. So the priests and monks that were trying to win over the pagans learned about the pagan holidays and tried to show similarities between their holidays. Give that comparison a few hundred years and it became well-known to include both as one holiday.
In addition, many priests actually became more paganized (is that a word?). The Dark Ages actually has a lot of missing information, but this is only what I remember from college. The Black Plague and the many revolutions, invasions, wars, etc. also make it difficult to obtain a lot of facts.
The birth of Christ was actually sometime in October. But, the huge celebration/feast that occurred during the winter solstice was too hard to pass up to be the celebration for the birth of Christ. Personally, if it is a different day, I don't think He'd mind. He'd be excited just to know his birthday was being celebrated. But the Christmas tree...pagan. Santa Claus...that's not Christian. Holly and yule log...pagan.
Halloween isn't even a Christian holiday. It's completely pagan.
Vance, you're right about Easter, too. The Easter bunny represented fertility and the revival of life. Eggs are also a symbol of fertility.
But think about it, if you were a priest living amongst pagans, you'd want to make friends, learn about eachother, share everything...it became a sort of compromise to where everyone was happy.