Well yes, it's true we celebrate the birth of Jesus Christ at Christmas time. But even more meaningful is the fact that we celebrate the Incarnation. Ya, that's a big church word but it really means that God came to town. He revealed Himself. He made Himself known in the best way we can grasp it, as a person.
My favorite Biblical rendering of the Christmas narrative (apart from Linus reciting Luke 2 from the ole KJV) is from John 1. No, there are no mangers or angels or shepherds or little babies. But there is the simple, yet most profound, explanation that "the Word became flesh and made His dwelling among us."
So, yes, at this Christmas, I wish you a Merry Christmas. But at this Christmas and throughout eternity, I wish you a Merry Incarnation...and understanding that the very being of God has been made known in Jesus Christ. O come, let us adore Him!