Today’s solemnity is normally celebrated on March 25; however, it was transferred because that date fell during Holy Week.
In the fifth century, bishops engaged in a fierce theological debate over the unity of the divine and human natures of Christ, referred to as the “hypostatic union.” Nestorius, the Archbishop of Constantinople, argued that there were two underlying hypostases, or substances, in Christ, one human and one divine. He believed that the humanity of Christ was born of the Virgin Mary, but not His divinity. Therefore, Nestorius believed Mary should only be called the “Christ-bearer” and not the “God-bearer” (Theotokos). Saint Cyril, the Archbishop of Alexandria, argued that the divine and human natures of Jesus were united in one hypostasis. Divinity and humanity were perfectly united and, therefore, Mary is properly called the Theotokos, or “Mother of God,” not just the “Mother of Christ.” God was born of her, not just His body. Mary conceived a Person in her womb, and that Person was both God and man, perfectly united as one. In 431 the Church held a council at Ephesus at which Cyril’s position was adopted and Nestorius’ position was condemned. Nestorius was then removed as Archbishop of Constantinople.
The solemnity we celebrate today originated around the time of this controversy, possibly to emphasize the theological teaching that emerged from the Council of Ephesus. Throughout Church history, when a theological truth is defined, that truth is then celebrated liturgically as a lived expression of the Church’s faith.
The date of this celebration is set nine months before Christmas, but most likely, the date of Christmas was set nine months after the Annunciation. Many early Christians believed that Jesus’ death occurred on March 25; therefore, His Incarnation must have also taken place on this day. As early as the third century, it was suggested that March 25 was also the date of the creation of Adam, the fall of Adam, the fall of the angels, the sacrifice of Isaac by Abraham, and the passing of the Israelites through the Red Sea.
The Solemnity of the Annunciation of the Lord recalls the historical event when the Angel Gabriel appeared to the Virgin Mary, as found in Luke 1:26–38. It also celebrates the Church’s faith in the underlying reality that took place at that moment in time. It celebrates the Incarnation, making this solemnity not only a Marian solemnity, but first and foremost a Christological one. Though Christmas commemorates the birth of Christ, the Annunciation commemorates His Incarnation.
Most certainly, this solemnity is also a Marian celebration. We celebrate the Virgin Mary’s perfect “Yes” to the plan of the Father. We celebrate her humility, obedience, docility, and especially her motherhood of God and man.
Finally, this solemnity is a celebration of the Father in Heaven because it reveals the beginning of the pinnacle of His perfect plan of salvation. That moment in time, the moment of the Incarnation, holds significance beyond comprehension. This moment was in the Father’s mind from all eternity and took place in a hidden way, known only to this lowly and humble virgin.
Source: https://mycatholic.life/saints/saints-of-the-liturgical-year/march-25-annunciation-of-the-lord/
