This feast dates back to the fifth or sixth century and like most Marian feasts was celebrated in the East prior to the West.
St Andrew of Crete writes a beautiful sermon for this day. He says that "so glorious a visitation of God to men needed some prelude of joy." He goes on to say that today there is celebrations both in heaven and on earth.
The birthday of Mary reminds us that every birth-day is a day of great joy. Paul VI says that a child is God's supreme gift to a couple. Mary was a gift to her parents, Joachim and Anna, who were without child. Every child is a gift from God and their future is unknown just as the glorious future of Mary was not known to her parents.
St Andrew finishes his sermon with, "today a shrine is built for the creator of the universe. The creature is newly made ready as a divine dwelling for the Creator."
What a wonderful feast.