Distorting the Gospel
"The second reason why such holy days are an abomination to God is that it is necessary to sanction error in order to give them our esteem. We shall again cite Christmas by way of example. If there were any possibility that the date of Christ's birth were preserved through tradition, then it would be January 6th rather than December 25th which deserved the preference. The Greek Church is an older institution than is the Latin. And if tradition has any validity, that validity depends upon antiquity.
"Even if we were to appeal to the false criterion of tradition we would be condemned! However, as tradition is condemned by Scripture we can neither build upon it nor be judged by it.
"Much more important is the fact that the celebration of Christmas (and other such humanly devised holy days) distorts the true gospel of Jesus Christ. By the special religious observance of certain days, certain aspects of the gospel are given a prominence which is not given them in the teaching of the Word of God itself. Christmas and Easter are the two “holy days” that claim an inordinate amount of attention each year, and so the birth and resurrection of Christ receive a measure of attention which other aspects of the truth do not receive.
"This emphasis is not found in the Apostolic writings. For in all the epistles of the New Testament we can discover no explicit reference to the so-called Christmas story. The resurrection of Christ does constantly receive much emphasis, but there is also much emphasis in the Apostolic writings on events which took place on other days that men have not memorialized with special days. This is a distortion of the truth of the Gospel, and a distortion of the truth is not the same as the truth itself. Thus to approve of such holy days we must approve of that which must be called error."
(Excerpt from Holy Days of Men and Holy Days of God by G.I. Williamson, posted at Reformed.org)