While I would accept that the date of Christmas was nothing to do with the Winter Solstice, it was, nevertheless, a clear appropriation of a pagan festival by the Western Church. This is set out by Sir James Frazer in his gargantuan work, The Golden Bough.
“The Gospels say nothing as to the day of Christ's birth, and accordingly the early Church did not celebrate it. In time, however, the Christians of Egypt came to regard the sixth of January as the day of the Nativity, and the custom of celebrating the birth of the Saviour on that day gradually spread until by the fourth century it was universally established in the East. But at the end of the third or the beginning of the fourth century the Western Church, which had never recognised the sixth of January as the day of the Nativity, adopted the twenty-fifth of December as the true date.”
Frazer goes on to quote an early Syrian Christian writer, who states, “It was the custom of the heathen to celebrate on the twenty-fifth of December the birthday of the Sun, at which they kindled lights in token of festivity. In these solemnities and festivities the Christians also took part. Accordingly when the doctors of the Church perceived that the Christians had a leaning to this festival, they took counsel and resolved that the true Nativity should be solemnised on that day and the festival of the Epiphany on the sixth of January.”
If this isn't the appropriation of a pagan festival, I'm not sure what is.
