Mother of my Lord.
"Most blessed are you among women, and blessed is the fruit of your womb. And how does this happen to me, that the mother of my Lord should come to me?" says Elizabeth. Elizabeth is surprised that Mary should come to her. Who am I, she says, that the Mother of God should come to me! In exclaiming this, Elizabeth shows great humility, for she recognizes that something great and mysterious is happening. Mary has no ordinary child, but she carries in her womb a single person who is both man and God. This is why she calls Mary "Mother of my Lord." Jesus is one person who is both human and divine. He isn't part human and part God, but He is fully both. Mary has one child, and this child is both fully man and fully God; hence, she can truly be called the mother of God, the mother of our Lord.
Mary, pregnant with the baby Jesus, is carrying the Son of God, the second person of the Blessed Trinity. This privilege belongs wholly and solely to Mary, and for this reason "all generations will call [her] blessed." Elizabeth is quick to realize this when she says, "Most blessed are you among women." Most blessed, she says. There is no woman greater than the virgin Mary. She outstrips all the rest. St. Thomas Aquinas would go so far as to say that not even God could create a woman more perfect than Mary.
Mary is most blessed on account of her Motherhood. Having predestined Mary to conceive the Incarnate Word, God lovingly prepared his Mother. He made her a fit dwelling place for Himself. By preventing her from contracting any stain of sin, He saved her by her Immaculate Conception. Mary was conceived without sin so that she could be a fitting house of the Lord. God wasn't forced to do this; it wasn't necessary. God gave Mary this great gift by His own free choice. It was purely on account of His love for the blessed Virgin that He prepared her to be His mother.
This blessed woman, the Mother of God, is a cause for joy. The infant in Elizabeth's womb, St. John the Baptist, leapt for joy when he heard Mary's greeting. Judging by Elizabeth's humble and gracious greeting it surely isn't a stretch for us to believe that Elizabeth was also filled with a great joy in being in the presence of Mary.
However, more important is God's own response to Mary. This is what the first reading is about. Daughter Zion is a title that most fittingly is applied to Mary. Jerusalem and Zion are both types which point to the blessed virgin. Consider the passage from Zephaniah, "O daughter Jerusalem! The Lord has removed the judgment against you, he has turned away your enemies; the King of Israel, the LORD, is in your midst, you have no further misfortune to fear." Mary, most fully, had the Lord in her midst. Mary, most fully, had no misfortune to fear, for God had turned all sin away from her. None of her enemies could touch her. Zephaniah continues, "He will rejoice over you with gladness, and renew you in his love, he will sing joyfully because of you, as one sings at festivals." God, Himself, sings and delights in Mary.
In Mary, there is much to delight. She is most blessed among woman. She is the Lord's lowly servant. She exalts the greatness of the Lord. She believes in the Word that is spoken to her by the Lord. These virtues are real causes for joy, and in whomever these are found God rejoices. Today, therefore, let us also join God and sing joyfully and rejoice in Mary who brought to us the LORD into our midst.