Nine months ago, Mary was immaculately conceived in the womb of her mother, St. Anne, by her father St. Joachim. The Feast of that Immaculate Conception, 8 December, is a much greater Feast than today's (it's a Holy Day of Obligation, in fact); but we recall Mary's birthday, too -- the birth of the woman destined by God from the beginning of time to be born of the House of David and the Tribe of Judah, the women whose enmity toward Satan was spoken of as far back as Genesis, the woman whom St. John saw crowned with stars and with the moon at her feet, the woman whom God chose to bear His Son and bring life to the world. With today's Feast, the line between the Old and New Testaments has been crossed; the New Covenant is imminent!
The Feast is one of the only three birthdays honored in the liturgical year (the others being that of St. John the Baptist and that of Jesus Christ Himself, all three born without original sin, though only Mary and Jesus were free from sin at the moments of their conceptions). We know little about Mary's birth and youth, most of our information coming from the apocryphal Gospel of the Nativity of Mary (translated from the Hebrew by St. Jerome, A.D. 340-420), the Protevangelium of St. James (dated to ca. A.D. 125), and the visions of various mystics through the years.
In celebrating the nativity of Mary, Christians anticipate the Incarnation and birth of her Divine Son, and give honor to the mother of Our Lord and Savior. The birth of Mary was also miraculous. She was conceived without sin as a special grace because God had selected her to become the mother of His Son (the feast of her Immaculate Conception is celebrated on December 8).