We believe that Christ entered the world after the Archangel Gabriel announced to Mary what was to take place. In fact (as you all know):
Luke1:35 And the angel said to 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.
So when Jesus was born, he had already been human for around 9 months earlier, therefore he was already incarnated then.
I myself celebrate Christmas “more” than the Annunciation, in fact I wasnt even sure that I attended Mass on that day (unless it was on a Sunday, which it was this year).
If I remember correctly, I once heard someone say on tv that Christmas was a time when we celebrate the start of our salvation. But isnt it more accurately said that our salvation started either at the conception of Jesus or the start of the Passion, not the birth of Jesus?