Translated by Ch. Browne and J. Swallow.
The Title of this Oration has given rise to a doubt whether it was preached on Dec. 25, 380, or on Jan. 6, 381. The word Theophania is well known as a name for the Epiphany; which, however, according to Schaff,  was originally a celebration both of the Nativity and the Baptism of our Lord. The two words seem both to have been used in the simplest sense of the Manifestation of God, and certainly were applied to Christmas Day. Thus Suidas, "The Epiphany is the Incarnation of the Saviour;" and Epiphanius (Haer., 53), "The Day of the Epiphany is the day on which Christ was born according to the flesh." But S. Jerome applies the word to the Baptism of Christ; "The day of the Epiphany is still venerable; not, as some think, on account of His Birth in the flesh; for then He was hidden, not manifested; but it agrees with the time at which it was said, This is My beloved Son (In Ezech. I.). There is also a Sermon, attributed to S. Chrysostom, "On the Baptism of Christ," in which it is expressly denied that the name Theophany applies to Christmas. The Oration itself, however, contains evidence to shew that the Festival of our Lord's Birth was kept at the earlier date; for in c. 16 the Preacher says, "A little later you shall see Jesus submitting to be purified in the river Jordan for my purification." And another piece of evidence occurs in the oration In Sancta Lumina, c. 14, "At His Birth we duly kept festival, both I the leader of the feast, and you. Now we are come to another action of Christ and another Mystery."
The Oration is thus analysed by Abbe Benoit:
"After an exordium which is full of the enthusiasm and joy which such a subject naturally inspires the Orator recommends his hearers to celebrate the Festival by a pious gladness, and by hearing the Word of God; and not as the heathen celebrated their feasts, by profane amusements and all kinds of excess. He will try to satisfy their desires by speaking to them of God. God is infinite, ineffable, eternal, the Sovereign Good. He created the Angels in the beginning out of goodness. The fall of the Angels was followed by the creation of the material world. Man too fell, and God shewed His mercy even in the punishment. He used various means to raise him again; and at length He came Himself. Then the speaker forcibly argues against those who misuse the infinite condescension of the Word to contest His Godhead; he rapidly traces the principal features of His Life—at once human and Divine; and ends with a recommendation to his hearers to imitate in all things the Life of Christ, so that they may have a share in His Kingdom in Heaven."
It is considered one of the best of Gregory's discourses. "By the grandeur of the plan," says Benoit, "the elevation of the ideas, and the rich fund of doctrine, this discourse is incontestably one of S. Gregory's most remarkable efforts."
 H. E., Nic. Per., p. 399.
Reference address : https://www.elpenor.org/gregory-nazianzen/birthday-christ.asp