This site provides historical information about the holiday and its development.
I've posted this sermon selection in previous years: "Sadness should have no place on the birthday of life. The fear of death has been swallowed up; life brings us joy with the promise of eternal happiness.
"No one is shut out from this joy; all share the same reason for rejoicing. Our Lord, victor over sin and death, finding no person free from sin, came to free us all. Let the saint rejoice as he sees the palm of victory at hand. Let the sinner be glad as he received the offer of forgiveness. Let the pagan take courage as he is summoned to life.
"In the fullness of time, chosen in the unfathomable depths of God's wisdom, the Son of God took for himself our common humanity in order to reconcile it with its creator. He came to overthrow the devil, the origin of death, in that very nature by which he had overthrown humankind.
"And so at the birth of our Lord the angels sing in joy: Glory to God in the highest, and they proclaim peace to his people on earth as they see the heavenly Jerusalem being built from all the nations of the world. When the angels on high are so exultant at this marvelous work of God's goodness, what joy should it not bring to lowly hearts?
"Beloved, let us give thanks to God the Father, through his Son, in the Holy Spirit, because in his great love for us he took pity on us, and when we were dead in our sins he brought us to life with Christ, so that in him we might be a new creation. Let us throw off our old nature and all its ways and, as we have come to birth in Christ, let us renounce the works of the flesh... Do not forget that you have been rescued from the power of darkness and brought into the light of God's kingdom."
(From a sermon by Pope Leo the Great, quoted in The Liturgy of the Hours, I, Advent Season and Christmas Season, pp. 404-405. I made the language inclusive in three places.)