Why do we read that Angels appeared at the time of the Birth of the Lord, but we read not that they appeared in white apparel whereas, when the Lord ascended into heaven, it is written that the angels which appeared were clad in white.
[...] White raiment is an outward sign of solemn inward joy. That the occasion of God-made-Man entering into heaven was a great Festival for Angels, is the reason which we see why angels are specially named as robed in white at His Ascension, and not at His Birth.
At the Birth of the Lord the Godhead was manifested veiled under the form of a servant, but at His Ascension the Manhood was seen exalted and white vestments are more apt to exaltation than humiliation.
[...] At His Birth He Who thought it not robbery to be equal with God, was seen in the form in which He had humbled Himself. At His Ascension the Manhood Which He had taken into God was seen glorified.
Again, dearly beloved brethren, we must remember to-day, how that Christ hath blotted out the hand-writing that was against us, and reversed the sentence which doomed us to corruption.
That same nature to which it was said “Dust thou art, and unto dust shalt thou return,” that same nature is His Who hath this day ascended up into heaven.
It is because of this up-lifting of our flesh that blessed Job, by a figure, calleth the Lord a bird. The Jews could not understand the Mystery of the Ascension, and in view of this their unbelief, blessed Job said mystically “He knew not the path of the bird.”
The name of a bird is well given to the Lord, Who bodily soared up into heaven. And the path of that Bird knoweth no man, who believeth not in the Ascension into heaven.
It is of this glorious occasion that the Psalmist saith Who hast set thy glory above the heavens (8:2), and again God is gone up with a shout, and the Lord with the sound of a trumpet (46:6). And yet again he saith Thou hast ascended on high, Thou hast led captivity captive (67:19).
When Christ ascended up on high, He led captivity captive, (Eph. 4:8), because by His Own incorruptibility He swallowed up our corruptibility.
He gave gifts unto men, because by sending the Spirit from above, He gave“to one, the word of wisdom to another, the word of knowledge to another, the working of miracles to another, the gifts of healing; to another, divers kinds of tongues to another, the interpretation of tongues, (1 Cor. 12:8-10).
Gregory the Great (c.540-604): Homilies on the Gospels, 29, from Mattins of Wednesday in the Octave of the Ascension in the Old Breviary @ http://lzkiss.net/cgi-bin/horas/brevi.pl.