Only Christ, St. John the Baptist, and the Virgin Mary have feasts commemorating their births, because only they were holy from the start. They were sanctified by God in the womb, not made holy through grace and long trial during their earthly lives.
Nowhere in Scripture is the place and date of birth of the Virgin Mary recorded. Nor are the names of her parents found in Scripture, although tradition tells us they were Joachim and Anne. It is not until the sixth century that there is certain knowledge of a liturgical commemoration of Mary’s birth. This is not unusual. Mary lived a largely hidden life, and her theological and historical significance remained somewhat veiled until the Council of Ephesus in 431 formally declared her the Mother of God. Since that definition, every aspect of her life has become the source of a rich spiritual and theological heritage.
While there are no written sources stating when exactly Mary was born, the date of her nativity was decided in comparison to the Feast of the Immaculate Conception. The latter is celebrated on the 8th of December making the two feasts exactly 9 months apart from each other.