How do we know that September 8 is the Blessed Mother's birthday?

[CindyGia]

I’m a new Catholic (confirmed two years ago) and was wondering if there are documents that confirm that the birth of our Lady is September 8. If not, how does the Church come to the conclusion that September 8 is the birthday of our Blessed Mother?

 

[Michelle_Arnold]

So far as I know, there is no documentation confirming September 8 as the actual birthday of our Lady. Usually, the Church assigns dates for particular feasts. The assignments can be based on the date of a saint’s death, on a date that has special meaning in the saint’s life, or on some other meaningful date. In the case of the Nativity of the Blessed Mother, September 8 is the date that follows nine months after December 8, on which date the Church celebrates the feast of the Immaculate Conception of Mary.