Dave, like most atheists, knows more about the history of Christianity than your average Christian. And so Dave knows that the traditional dating of the birth of Jesus is very likely inaccurate and that it was probably chosen by the church establishment to coincide with pre-Christian religious ("pagan") ceremonies such as sun-worship and solstice observance. And of course, on a less combative note, Dave understands the science behind seasonal changes and the tilt of the earth. But I do suspect he's engaging in a bit of good-natured teasing of us Christians, and so I'd like to respond.
However unlikely it may be that December 25 was the actual birthdate of Jesus, what is not in dispute is that Jesus was a real person of enormous historical significance. (Only the fringiest of fringe historians disputes the historicity of Jesus; it would be roughly equivalent to the number of scientists who believe global warming is a hoax or that the evolution did not occur.) And regardless of what one may think of Jesus' many claims to be God—the fact is that at this time of year, large numbers of people in all parts in the world and in nearly every culture do celebrate his birth. The overwhelming majority of us sing December songs to the newborn King, not Mithras or Ra.
So yes, in one sense, solstice is the "reason" for our observance of Christmas this month instead of, say, September or May. But it is a frail trick of semantics to suggest that the season is anything other than a joyous, worldwide celebration of the birth of Jesus, emmanuel, "God with us."
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"The light shines in the darkness, and the darkness did not overcome it." John 1:5