Ascension is to Christmas what Theosis (or Deification) is to the Incarnation.
Ascending on high, he led captivity captive; he gave gifts to men. Now that he ascended, what is it, but because he also descended first into the lower parts of the earth? He that descended is the same also that ascended above all the heavens, that he might fill all things.
God descended that Man might ascend.
God came down so that Man might go up.
Where the Head is their shall the Body be.
He shared in our humanity so that we migh share in his divinity.
He became Man so that we might become God.
He became sin (which we are by virtue of our fallen humaity) so that we might become holy (which he is by virtue of his divinity)