The Bible is silent about the birth of the Virgin Mary, but the Church celebrates it annually on September 8.
This day is universally celebrated in both the East and the West, originally found in the calendars of Eastern Christians.
According to the Catholic Encyclopedia, “The earliest document commemorating this feast comes from the sixth century. St. Romanus, the great ecclesiastical lyrist of the Greek Church, composed for it a hymn … The feast may have originated somewhere in Syria or Palestine in the beginning of the sixth century, when after the Council of Ephesus, under the influence of the ‘Apocrypha,’ the cult of the Mother of God was greatly intensified, especially in Syria.”
Read more at Aleteia