Second Day - Et concepit de Spiritu Sancto ("And she conceived of the Holy Ghost"): Christi autem generatio sic erat: cum esset desponsata mater ejus Maria Joseph, antequam convenirent inventa est in utero habens de Spiritu Sancto. (St. Matthew, i, 18: "Now the generation of Christ was in this wise. When as his mother Mary was espoused to Joseph, before they came together, she was found with child, of the Holy Ghost."). Et respondens angelus dixit ei : Spiritus Sanctus superveniet in te, et virtus Altissimi obumbrabit tibi. Ideoque et quod nascetur ex te sanctum, vocabitur Filius Dei. (St. Luke, i, 35: "And the angel answering, said to her: The Holy Ghost shall come upon thee, and the power of the most High shall overshadow thee. And therefore also the Holy which shall be born of thee shall be called the Son of God.")
..to be born of a woman indeed was our lot, but "to be born not of blood, nor of the will of flesh, nor of man," but of the Holy Ghost, was to proclaim beforehand the birth surpassing us, the birth to come, which He was about freely to give us of the Spirit. And everything else too was like this. Thus His baptism also was of the same kind, for it partook of the old, and it partook also of the new. To be baptized by the prophet marked the old, but the coming down of the Spirit shadowed out the new. And like as though any one were to place himself in the space between any two persons that were standing apart, and stretching forth both his hands were to lay hold on either side, and tie them together; even so hath He done, joining the old covenant with the new, God's nature with man's, the things that are His with ours.
Saint John Chrysostom, Homilies on the Gospel according to Saint Matthew
The Holy Spirit will come upon you, behold, the heat [of the sun]! Heat sustains and nourishes all living things, and the lack of it results in weakness and death. Death is the extinction of natural heat in the heart, due to a lack of moistness and the coming of its contrary. The cause of leaves falling is the removal of the heat that feeds them. In winter-time, when cold dominates the outside of trees and plants, the heat flees from its contrary and hides in the roots. When it increases there, it draws to itself moisture from the upper branches and extremities, so that this increase in heat is moderated and the lower parts do not burn. When their sustenance is withdrawn, the leaves are bound to fall from the upper parts. The grace of the Holy Spirit is ‘heat’, and if it withdraws from the human heart the moisture of compunction fails, and the unhappy soul falls into the depths of sin. When the cold of wickedness approaches, the heat of the Holy Spirit flees from its contrary, and so the soul is despoiled of all its goods. The entry of vice brings about the departure of virtue. So the book of Wisdom says:
"The Holy Spirit of discipline will flee from the deceitful,and will withdraw himself from thoughts that are without understanding; and he shall not abide when iniquity cometh in." [Wisdom, i, 5]
That is to say, he is snatched away, together with all his goods, because of the entry of wickedness. But when heat returns, the earth conceives and germinates plants, and brings forth fruit. Just so, when the Holy Spirit came, the blessed earth conceived and gave birth to the blessed fruit, removing every curse. Yes, The Holy Spirit will come upon you. Blessed Mary was ‘as the shining sun’ at the angel’s Annunciation.
Saint Anthony of Padua, Sermon on the Annunciation