This is known as the Feast of the Impossible! It is a mystery that is at the heart of our faith. An angel came to Mary and announced that she had found favor with God and that she would bear a son and he would be great and would be called the Son of the most high and "the Lord God will give him the throne of David his father, and he will rule over the house of Jacob forever, and of his Kingdom there will be no end." Imagine how Mary felt on hearing this message! She is perplexed and asks the angel how can this happen? And the angel tells her, "The Holy Spirit will come upon you, and the power of the Most High will overshadow you. Therefore the child to be born will be called holy, the Son of God." We celebrate this mystery every time we say the "Hail Mary" and we can never thank Mary enough for having the courage and love to say, "Behold, I am the handmaid of the Lord. May it be done to me according to your word."
We are called to say our "yes" to whatever the Lord asks even if we do not understand.