St Gregory the Dialogist, in Homily 21 ("The Mystery of the Resurrection"), writes:
The women who came with spices saw the angel seated on the right side. What does the left side mean except this present life, and what does the right side mean except eternal life? Hence it is written: 'His left hand shall be under my head, and His right hand shall embrace me' [Song 2:6]. Since our Redeemer has already passed beyond the corruption of the present life, it was right that the angel who had come to proclaim His everlasting life should be seated on the right side.Passages such as this remind me of just how richly imaginative (in the best sense of that word) and daring the writings of the Fathers so often are. Who would speak in such terms today? Even the best modern sermons seem awfully thin and tame in comparison.
The angel appeared in a white robe to proclaim the joy of our festive day, since the whiteness of the garment declared the splendor of the solemnity. Should I say our solemnity and the angel's? To speak truly I must say that it belongs to both. Our Redeemer's Resurrection was our festival day because it led us back to immortality, and also a festival day of the angels, because by recalling us to the things of heaven it completed their number. Therefore on our festival days, and theirs, an angel appeared in white garments, because our restoration to the things on high by the Lord's Resurrection makes up for the losses of the country of heaven.