In a previous post, I considered the question whether God would have incarnated had humanity never sinned. The answer I gave there was an affirmative one: yes, God would have incarnated anyway because his goal from the beginning was to incarnate and be among human persons. God's decision from the beginning was to be together with humanity as the theanthropos, the GodMan.
Now it seems to me that Christmas Day, in which we celebrate the birth of Christ, has important connections to the notion of the Kingdom of God for this reason. As I understand it, the Kingdom of God is that state or condition of things according to which God's will is being realized. Thus, the Kingdom of God means that things are going well for his creation, since he is good and benevolent; it means that humanity is obeying him and enjoying him and worshiping him, as well as living in peace with one another. The Kingdom of God, to my mind, means the sovereignty or rule of God, it means that what God wants is taking place.
If God's intention from the beginning was that he be among men and women in the incarnation, then on Christmas Day we can say that the kingdom of God has come near (Mark 1.15). This was Jesus' gospel proclamation, this was the good news that he brought to the people of Israel at that time: that the kingdom of God had come near to them, and that finally God was taking control of things and setting everything right.
But the Kingdom of God doesn't consist only in the healings, in the teaching, in the fellowship and fraternity of humanity, but also in the incarnation. The newborn baby crying in his mother's arms, the parents looking upon this miracle child of theirs, the young boy Jesus growing up and learning to walk and to speak and to read from Torah -- all of this is the Kingdom of God, too. The Kingdom of God was present with Mary and Joseph and the rest of Jesus' family from the very beginning; it accompanied them his entire life until death and afterwards, as well.
So when we celebrate Christ's birth, we are not celebrating some necessary precondition for the Kingdom later to appear. No, with the very conception of Christ and his birth, the Kingdom is already present and has already begun.