This Christmas, I read through Gregory of Nazianzus’ 38th Oration, On the Theophany. It is wondrously beautiful. Gregory’s theology and language meld into one lovely, harmonious whole. I hope reading through this oration becomes a Christmas tradition! I’ve worked up a little poem to share here. It is a verse rendition of the beginning of the oration. I’m not a particularly good poet, but hopefully enough of Gregory comes through to make it enjoyable. Fr. Aidan posted an English translation of the entire oration here, which you may also view at New Advent.
Theophany I
All ye, come meet the Christ and sing to God,
Thou Plenitude of Earth. Yet I must name
the both: let hea’ens and earth be glad
and make much cheer, Uranic Splendor came,
assumed our terran shame, and in flesh lay.
O man, rejoice in fear, in joy rejoice!
In fear for sin, in joy for hope of him:
The Christ-child borne of Virgin womb and shame!
O Eve’n Daughters, those of Adam’s race,
do now take up your virgin pur’ty, O
that ye be little Mary’s, full of Christ within.
Who shan’t praise him, the Chosen One who comes
of the beginning? Who shall not raise his voice
to him in whom our being finds finality?
Here is the Greek. For my fellow hellenists, much of the language in this oration is pretty simple. It gets difficult and theologically complicated at points, but a good bit is not all that difficult. My way of saying, this is recommended reading! The Greek text of the oration may be found here.
Χριστὸς γεννᾶται, δοξάσατε· Χριστὸς ἐξ οὐρανῶν, ἀπαντήσατε· Χριστὸς ἐπὶ γῆς, ὑψώθητε. ᾌσατε τῷ Κυρίῳ, πᾶσα ἡ γῆ· καὶ, ἵν ̓ ἀμφότερα συνελὼν εἴπω, Εὐφραινέσθωσαν οἱ οὐρανοὶ, καὶ ἀγαλλιάσθω ἡ γῆ, διὰ τὸν ἐπουράνιον, εἶτα ἐπίγειον. Χριστὸς ἐν σαρκὶ, τρόμῳ καὶ χαρᾷ ἀγαλλιᾶσθε· τρόμῳ, διὰ τὴν ἁμαρτίαν· χαρᾷ, διὰ τὴν ἐλπίδα. Χριστὸς ἐκ Παρθένου· γυναῖκες παρθενεύετε, ἵνα Χριστοῦ γένησθε μητέρες. Τίς οὐ προσκυνεῖ τὸν ἀπ᾽ ἀρχῆς; τίς οὐ δοξάζει τὸν τελευταῖον;
ἐν αὐτῷ,
ΜΑΘΠ
Alex Poulos, PhD 
You may be interested to know that, in the 8th cent., St. Cosmas the Melodist (brother of St. John Damascene) based his canon on the Nativity on this sermon. Thus, the opening lines of Gregory’s sermon are to this day still one of the most beloved Christmas hymns for Orthodox Christians. The first stanza is:
Χριστὸς γεννᾶται, δοξάσατε. Χριστὸς ἐξ οὐρανῶν, ἀπαντήσατε. Χριστὸς ἐπὶ γῆς, ὑψώθητε, ᾌσατε τῷ Κυρίῳ, πᾶσα ἡ γῆ, καὶ ἐν εὐφροσύνῃ ἀνυμνήσατε λαοί, ὅτι δεδόξασται.
This hymn is sung for weeks in advance of Christmas and, of course, on Christmas. It is so familiar to Orthodox Christians that often the first three words are used as a greeting similar to the paschal greeting, Χριστὸς ἀνέστη! Ἀληθῶς ἀνέστη! So often priests will greet their flocks with Χριστὸς γεννᾶται! and hear Δοξάσατε! in response. I think that this is more prevalent in the Russian tradition.
You can hear most of the canon on Youtube:
Actually, two canons are appointed for Christmas, and both are chanted on that video. Each ode has four stanzas from the first canon, then three from the second. Despite the Youtube title, this video skips some of the odes.
Text of the whole canon can be found here:
http://analogion.gr/glt/texts/Dec/25.uni.htm
Begins about half-way down the page.