The story of the Annunciation, meaning the announcing, from the Latin annuntiare, is told in Luke's gospel. At the Annunciation, the angel Gabriel announces to Mary that she will conceive a Son, and his name will be Jesus. His greeting, "Hail Mary, full of grace, the Lord is with you" has echoed down through the ages in many prayers, and is known as the "Hail Mary." Mary is initially confused as to how she will bear God's Son, seeing as she is a virgin. The angel then explains that the Holy Spirit will come upon on her. This is why when we recite the Nicene creed we say "by the power of the Holy Spirit, [Jesus] was born of the Virgin Mary and became man." The Apostles Creed likewise affirms that Jesus was "conceived by the power of the Holy Spirit." Thus, the Feast of the Annunciation is the beginning of Jesus' miraculous life, and it begins with the theotokos conceiving Jesus by the Holy Spirit's power.
Mary's response to the angel, "Behold, I am the handmaid of the Lord; let it be done to me according to your word," (Latin: ecce ancilla Domini; fiat mihi secundum verbum tuum) is a statement of humble faith, and a model for how we are to respond when God calls us to do what seems impossible. This response is called Mary's fiat, from the Latin word meaning "let it be done." The Catechism addresses the significance of Mary's faith in relation to her role as Christ's mother:
By pronouncing her "fiat" at the Annunciation and giving her consent to the Incarnation, Mary was already collaborating with the whole work her Son was to accomplish. She is mother wherever he is Savior and head of the Mystical Body (973).