J.M.J. "The origin of this Feast is sought in Palestine. It goes back to the consecration of a church in Jerusalem, which tradition identifies as that of the present basilica of St. Ann.
"At Rome the Feast began to be kept toward the end of the 7th century, brought there by Eastern monks. Gradually and in varied ways it spread to the other parts of the West in the centuries that flowed. From the 13th century on, the celebration assumed notable importance, becoming a Solemnity with a major Octave and preceded by a Vigil calling for a fast. The Octave was reduced to a simple one during the reform of St. Pius X and was abolished altogether under the reform of Pius XII in 1955. The present Calendar characterizes the Birth of Mary as a "Feast," placing it on the same plane as the Visitation.
"For some centuries now, the Birth has been assigned to September 8 both in the East and in the West, but in ancient times it was celebrated on different dates from place to place. However, when the Feast of the Immaculate Conception (which has a later origin than that of the Birth) was extended to the whole Church, the Birth little by little became assigned everywhere to September 8: nine months after the Solemnity of the Immaculate Conception."
--The Reverend Alberto Valentini, S.M., S.S.D., "Birth of Mary" in Dictionary of Mary: "Behold Your Mother" (New York: Catholic Book Publishing, 1985), 35-36.