And Mary said to the angel, “How will this be, since I am a virgin?”
And the angel answered her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be called holy—the Son of God."
Christmas is about the Incarnation of the Son of God, the breaking in to human history of Immanuel - God with us as one of us. Christmas is the basis of all our knowledge of God. I think that without it I would be an atheist, or at least agnostic. If God had not shown himself to us, in real space and time and history, I'm not sure I would have spotted the signs of his presence throughout creation.
But Christmas is also about the Holy Spirit, and his own pre-eminent work. Throughout Scripture, the Holy Spirit is portrayed as that person of the Godhead who is working within the creation, sustaining it and renewing it. And here he is sanctifying one part of genuine created reality for the greatest purpose of all: the coming in of God himself, not only to be near humanity but to be a human being. As Jesus is conceived by the Holy Spirit, so he will be empowered throughout his life by the Spirit, and will eventually offer himself through the same Spirit to the Father, suffering and dying, before being raised again by the power of that same Holy Spirit.
There is a sort of vague 'spirit of Christmas' which is abroad, a generic festivity and merriment. But the true Spirit of Christmas is very specific, very personal. He is the Spirit of Jesus, the man who is God. And that should mean more merriment, deeper festivity, real celebration.