The earliest record of Mary's birth is in the Gospel of James, which was written in about 145 AD. This tells of Mary's birth and childhood, and the nativity of Jesus. It's the oldest text to attest to Mary's virginity. Some believe that it was written by James, the brother of Jesus, but this has been shown not to be the case.
The Church commemorates the date of Mary's birth (rather than her death, as is the case with the saints) because her birth itself was holy; this is linked to the concept of the Immaculate Conception (the belief that Mary was free from original sin by virtue of the foreseen merits of her son Jesus Christ). The Catholic Church teaches that Mary was conceived by normal biological means in the womb of her mother, Saint Anne, but God acted upon her soul, keeping it "immaculate".
The only other person whose birth is commemorated in the Church, rather than his death (not counting Jesus, obviously), is John the Baptist.
© Haydn Thompson 2017