It’s Mary’s birthday. Very early in Christian history, the apocryphal gospels recorded legends of Mary’s birth and childhood. The Church has probably celebrated her birth with a feast day since at least the early fifth century. The feast may have originated in Syria, where there was a great flowering of Marian devotion in the fifth century, after the Council of Ephesus confirmed the orthodoxy of her ancient title “Mother of God” (Theotokos).
Among the Fathers, the earliest witness to the feast day is St. Romanos the Melodist (who flourished 536-556). Romanos is known as “the Pindar of rhythmic poetry,” and his hymns remain liturgical standards in the Byzantine liturgy. Born in Syria, he served as a deacon first in Beirut and later in Constantinople. (I’m pleased to say that my alma mater, Penn State, now houses a world-class scholar of Romanos, Dr. William Petersen, director of the University’s Religious Studies Program.) Romanos wrote several hymns for the feast, drawing heavily from the legends of the apocrypha. We also have, from two centuries later, St. Andrew of Crete’s sermons for the feast. The Church of Rome adopted the day in the seventh century; it is found in the ancient Gelasian and Gregorian Sacramentaries. Sergius I (687-701) prescribed a litany and procession for Mary’s birth.
UPDATE: Here’s some nice stuff on Mary’s birthday, including excerpts from those sermons of St. Andrew of Crete. I found the link in a comments field at Father Z’s place.