This great festival takes its name from the happy tidings brought by the angel Gabriel to the Blessed Virgin, concerning the Incarnation of the Son of God.
It commemorates the most important embassy that was ever known: an embassy sent by the King of kings, performed by one of the chief princes of His heavenly court; directed, not to the great ones of this earth, but to a poor, unknown virgin, who, being endowed with the most angelic purity of soul and body, being withal perfectly humble and devoted to God, was greater in His eyes than the mightiest monarch in the world.
When the Son of God became man, He could have taken upon Him our nature without the cooperation of any creature; but He was pleased to be born of a woman. In the choice of her whom He raised to this most sublime of all dignities, He pitched upon the one who, by the riches of His grace and virtues, was of all other the most holy and the most perfect.
The design of this embassy of the archangel is to give a Savior to the world, a victim of propitiation to the sinner, a model to the just, a son to this Virgin, remaining still a virgin, and a new nature to the Son of God, the nature of man, capable of suffering pain and anguish in order to satisfy God's justice for our transgressions.
-Little Pictorial Lives of the Saints