St. Photius, Patriarch of Constantinople gave a sermon in the late 9th Century on the Feast of the Annunciation which celebrates the events described in Luke 1:24-38. He reveals in his words the theological and cosmic importance of the Feast.
“Gay is today’s festival, and splendid is the joy it conveys to the ends of the earth. The joy it yields banishes the cures of the world, inaugurates the raising of him who fell long ago, and pledges salvation to all of us. An angel converses with a virgin, and the whispering of the serpent is made idle, and the impact of his plot is averted. An angel converses with a virgin and Eve’s deceit fails, and convicted nature, seen to rise about condemnation, as it had been before condemnation is enriched with the possession of paradise as its portion. He speaks to the Virgin, and Adam receives a pledge of liberty, and the serpent, instigator of evil, is deprived of his tyranny over our kind, and is dispossessed of his authority, and learns now that he had armed himself in vain against Creation. His devices against us weaken, as an incorporeal being brings the message of the invincible trophy against sin: for Christ’s cross and willing suffering are death and sin swallowed up in victory, and such also is His suffering through the Incarnation.
The angel is now bearing the good tidings of the Incarnation, in which tidings we are rejoicing today, and whose festival we are celebrating. An angel is being sent to the Virgin, and human nature is renewed; for, having quaffed the tidings like a remedy of salvation, it spits out all the poison of the serpent, and is cleansed from the spots of its disease. An angel is being sent to the Virgin, and the bond of sin is being torn up, and the penalty for the disobedience is abolished, and the universal recall is pledged in advance. Today the tidings of joy have arrived, since the archangel is exchanging words with the virgin maiden, the commander of the invisible host is conversing with Mary, espoused to Joseph but designated and preserved for Jesus. (The Homilies of Photius Patriarch of Constantinople, p. 112-113)
An historical note about March 25 and the Annunciation:
“…the origin of the solemnity on the fifth Saturday of Great Lent is linked to a shift in the feast of Annunciation. We know that, in the spirit of Canon 51 of the Council of Laodicea (c. 365), it was not appropriate to celebrate feasts during Great Lent, and such celebrations consequently had to be shifted to the following Saturday or Sunday. It was only the Council in Trullo (692) that decided to celebrate Annunciation on the very day (March 25).” (Archimandrite Job Getcha, The Typikon Decoded, p. 200)