The most ancient sources call this feast Annunciation (euangelismos) of the Mother of God. The official title Annunciation of the Lord underlines that this day has a character of a feast of the Lord.
The gospel of Luke 1:26-38 narrates the story of the angel declaring to Mary: “Do not be afraid, Mary, for you have found favor with God. You will conceive in your womb and bear a son, and you will name him Jesus…” Mary replied: “Here am I, the servant of the Lord; let it be with me according to your word.”
After the Birth of Jesus was established on December 25 in most Eastern Churches, the annunciation and conception of Jesus was fixed on March 25 during the 6th century. The western Church adopted this feast in the 7th century too and celebrated it along with other Marian feasts.
The liturgy offers readings from Isa (Behold, the virgin shall conceive), Hebr (As is written of me in the scroll, behold, I come to do your will, o God), and Luke (Behold, you will conceive in your womb and bear a son). The preface of this feast talks about Mary who lovingly bears in her immaculate womb Christ, the Eternal Word.
During the recitation of the Creed, all genuflect at the words was conceived by the Holy Spirit, born of the Virgin Mary, expressing reverence in front of the mystery of Incarnation.