Today we celebrate the feast of the Nativity of our Blessed Mother Mary. Perhaps you are asking yourself how we know the exact date of her birth. We don't. The feast was chosen as it is nine months after the feast of the Immaculate Conception. There is no mention of Mary's birth in the Bible. What we know about those early years has been obtained from sources "cited by some of the earliest Christian writers from the first centuries of the Church" (Catholic News Agency, 2012).
According to EWTN, the feast traces back to Palestine and "goes back to the consecration of a church in Jerusalem, which tradition identifies as that of the present basilica of St. Ann." This source also indicates that the feast began to be celebrated in Rome near the end of the 7th Century.