Yesterday I had a delightful meeting with a parishioner who was seeking ways to respond to Jehovah's Witnesses. He seems to have befriended one who has been "working on" him for some years.
Apparently one of the main attacks this Witness makes on ordinary Christianity is the observance of the "man made" holidays of Christmas and Easter. First of all, the days have to be considered separately.
Easter, though it perhaps corresponds to certain pagan festivals surrounding the vernal equinox, is intimately connected with the Passover, which is certainly part of divine revelation. God did not invite the children of Israel to observe the Passover; he commanded them to do so. If you want to believe the Gospels, the Last Supper was or was not the Passover meal, but in any case, the Passion, death, and Resurrection of the Lord is correlated with the Passover in a pretty direct way. So the assertion that the celebration of Easter is a "man made" or pagan observance has no merit.
Christmas is another story. Serious guesses at the birth date of Jesus of Nazareth place it perhaps in the spring of the year we would now call 3 or 4 BC. But it's anybody's guess, really. The commemoration of the Lord's birth, which we celebrate on December 25th, does in fact correspond to various pagan festivals associated with the winter solstice. Putting Christmas in their place assisted the conversion of culture to Christianity and fit astronomy to theology as we celebrate the re-birth of the light of the world. (At least in the Northern hemisphere.)
But do the pagan roots of observing Christmas when we do make it wrong and something to be rejected? And if not, why? Christians have always had the right and privilege to borrow what is helpful from the pagan world, e.g. culture, philosophy, etc., and adapt it to Christian purposes. But where do we get this right and privilege? It is founded on the Christian sense of creation and Incarnation.
The world comes to be through the Word of God. We see this in the prologue of the St. John as well as the first creation account from Genesis: "God said...and so it happened." We believe that this Word became flesh in Jesus Christ. Thus, Jesus is the revelation of the pattern on which all of the divine creative activity is based. Therefore, all created things--as well as art and culture, the grandchildren of God, as it were--somehow point back to the Word of God in their internal logic. We see this very clearly in ourselves in our heart's perpetual longing for love and beauty and intimate union; this is the creative force of God alive within us. From this we see that everything human, evangelized and adapted, can be put into the service of Christ.
Now I woudn't push this line of argument as far as Karl Rahner's famous "anonymous Christianity," which I find a little patronizing, but it does stand up to Sacred Scripture and it is certainly strong enough to justify plundering the winter solstice festivals from the pagans and transforming them into the commemoration of the Nativity of the Lord.