In this chapter Fr. Hopko tells us that the feast of the Entrance of the Theotokos in the Temple is the first time that the birth of Christ is announced during this season. He then explains that the canon of the Nativity of Christ was inspired by a famous homily given by St. Gregory the Theologian in the fourth century.
If you have a chance, read the homily linked above, because it is beautiful. It is so poetic and truly captures the joy of the feast. Here are a few excerpts:
Let the heavens rejoice, and let the earth be glad, because of Him who is of heaven and is now on earth. Christ in the flesh, rejoice with trembling and with joy; with trembling because of your sins, and with joy because of your hope.
... for He who gives riches becomes poor, for He assumes the poverty of my flesh that I may assume the riches of His divinity. He that is full empties Himself, for He empties Himself of His glory for a short while that I may have a share in His fullness.
... in creation He gave us a share of His own good nature. And now in the Nativity He takes on Himself our own sinful one.
The homily is full of this comparison of opposites and the tension between them: trembling - joy, rich - poor, empties Himself that I may have a share in His fullness...
Christ is born; glorify Him!
Christ comes from heaven; go to meet Him!
Christ is on earth; be exalted!
Sing to the Lord, all the earth!
And praise Him in gladness, O people,
For He has been glorified!
No wonder we have such great anticipation for this feast!