Beginning with the Feast of the Entrance of the Theotokos into the Temple on November 21 one can find hymns of the birth of Christ being sung at Matins during the season of the Nativity Fast. For example, from the November 21st Feast in Matins we sing:
CHRIST IS BORN, GLORIFY HIM;
CHRIST IS FROM HEAVEN, RECEIVE HIM.
CHRIST IS ON EARTH, BE LIFTED UP.
SING TO THE LORD ALL THE EARTH,
AND PRAISE HIM WITH JOY ALL PEOPLE,
FOR HE HAS BEEN GLORIFIED!
Some of the hymns, part of the various canons for different feasts, either quote or are based on the writings of St. Gregory the Theologian (d. 391).
“Christ is born, give glory; Christ is from the heavens, go to meet him; Christ is on earth, be lifted up. ‘Sing to the Lord, all the earth,’ and, to say both together, ‘Let the heavens be glad and let the earth rejoice,’ for the heavenly one is now earthly. Christ is in the flesh, exult with trembling and joy; trembling because of sin, joy because of hope. Christ comes from a Virgin; women, practice virginity, that you may become mothers of Christ. Who would not worship the one ‘from the beginning’? Who would not glorify ‘the Last’?
[…] The fleshless one takes flesh, the Word is made coarse, the invisible one is seen, the impalpable one is touched, the timeless one makes a beginning, the Son of God becomes Son of Man.
[…] This is our festival, this is the feast we celebrate today, in which God comes to live with human beings, that we may journey toward God, or return – for to speak thus is more exact – that laying aside the old human being we may be clothed with the new, and that as in Adam we have died so we may live in Christ, born with Christ and crucified with him, buried with him and rising with him.” (St. Gregory of Nazianzus, Festal Orations, pp 61-63)