From the Treasury of Daily Prayer:

The angel Gabriel appears to Mary and announces that God has shown her favor and will use her as the means for the Messiah’s birth. So Mary conceives Jesus when the angel says: “The Holy Spirit will come upon you, and the power of the Most High will overshadow you” (Luke 1:35). This same Spirit who hovered over the waters and brought forth creation (Genesis 1:2) will now “hover over” the waters of Mary’s womb to conceive the creation’s Redeemer. As the Holy Spirit comes upon Mary, she conceives Jesus “through her ear” (as Martin Luther says). The one who is conceived is called Holy, the Son of God. This is the moment of the incarnation of our Lord. The date of the Annunciation falls on March 25, because the Ancient Church believed the crucifixion occurred on that date. In antiquity, people linked the day of a person’s conception with the day of his or her death. Thus, in the Annunciation, the Church joined together both the incarnation of Jesus and the atonement He accomplished.