Our Lady's Birthday
I'm not sure when it was that September 8 was accepted as Our Lady's Birthday, but working backwards from that date for nine months we come to December 8, when we celebrate her Immaculate Conception.
Mary was declared to have been free from Original Sin from the very moment of her conception in the womb of her mother Anne. And it is eminently fitting for that to have been the case - she who was to become the Mother of God's only begotten, would necessarily have to have been free from sin from the very moment of her conception.
In contrast, John the Baptist was conceived with the stain of Original Sin on his soul, but it was removed before he was born, so that he who was to be later described by Christ in these words "Amen, I say to you, among those born of women there has been none greater than John the Baptist" was born free from Original sin.
From time immemorial, God had chosen the name "Mary" for this child who was to become the Mother of His only begotten, and so on the eighth day after her birth, a band of angels came down from heaven, bearing an escutcheon with the name "Mary" engraved on it, and showed the holy parents Joachim and Anne what her name was to be. Accordingly they decided immediately to name her that way, in obedience to God's will.
So September 8 is celebrated with Mass, and with the Rosary, and in various places Birthday parties are held for Our Lady on her birthday.