The date of September 8th was chosen as the 8th day after the Byzantine New Year, and the celebration of Mary’s birthday was celebrated widely in the Eastern Church beginning in the 6th Century with the dedication of a church to St. Anne on a site reputed to have been the childhood home of Mary. From the Eastern Church, this celebration of Mary’s birthday spread to the Western Church. Although Mary’s birthday has been celebrated on different dates, the predominant date has been September 8th. While the oldest existing hymn written for a liturgy of the birth of St. Mary dates from 560 AD and was written by Romano the Melodist, the oldest existing sermon recorded for the liturgy of her birthday was written by St. Andrew of Crete. The oldest known piece of artwork representing the birth of Mary dates from the 6th century.
There are only three birthdays that the Catholic Church celebrates: Mary’s, John the Baptist’s, and Christ’s. This is because the Church generally considers the date of death to be the more important date, since that is the date of entrance into Heaven for the saints. However, in the case of the three named above, she recognizes their birth dates as significant because without Mary to give Him birth, and John to point the way to Him, Christ’s saving work would not have been possible. What is truly celebrated, then, is the markers of God’s plan for salvation coming to fruition. Those who are familiar with the doctrine of the Immaculate Conception will note that the date of September 8th is precisely 9 months following the celebration of the Immaculate Conception, on December 8th.
Ways to Honor Mary's Birthday:
1) Pray a Rosary
Aside from attending Mass, praying the rosary is one of the best ways to honor Mary for it was she who presented the rosary to St. Dominic and it is she who repeatedly has called for the Christian faithful to pray it.
2) Attend Mass
There is no better way to honor Mary than to pay homage to her son. As with many mothers, the fastest way to win her love is to love her son.
3) Give a rose to a stranger
Mary is known as the Rose of Sharon, and the rose is her special symbol. Giving a rose to a stranger in honor of her birthday is one creative way to spread devotion to Mary to others.
4) Visit a neighbor
The first act of Mary's after discovering that she was carrying the Christ child was to go and visit her neighbor, Elizabeth. In visiting your neighbor, you are also carrying Christ to others and the fact that you do it in her name makes it all the more special to her.