Gregory the Great, on Acts 1:10

“And while they were beholding him going up to heaven, behold two men stood by them in white garments.”

“But first we must inquire what it means that when the Lord was born, angels appeared, yet they are not recorded as having appeared in white garments, whereas when the Lord ascended, angels who were sent are recorded as having appeared in white garments. For it is written thus: "While they were watching, he was lifted up, and a cloud received him from their sight. And while they were gazing into heaven as he went, behold, two men stood beside them in white garments." In white garments, joy and festivity of mind are shown. What does it mean, then, that when the Lord was born, angels appear not in white garments, but when the Lord ascended, angels appear in white garments, unless that then a great festivity was made for the angels, when God as man entered heaven? For when the Lord was being born, divinity seemed to be humbled; but when the Lord ascended, humanity was exalted. For white garments are more fitting for exaltation than for humiliation. Therefore at his assumption, angels ought to have been seen in white garments, because he who at his nativity appeared as humble God, at his Ascension was shown as exalted man.”

Imported from an open dataset — not yet checked against the printed edition.