In the post-1969 Roman Calendar, tomorrow's feast of the Nativity of the Blessed Virgin Mary will pass uncommemorated in the liturgy, thanks to the contemporary calendar's inexplicable allergy to multiple commemorations occurring on the same day. This is unfortunate, not only because of the great antiquity of tomorrow's feast, but because such ancient and once widely observed celebrations are nowadays noticed by virtually no one - except when they displace a Sunday as, for example, the Birth of Saint John the Baptist does every few years when June 24 falls on a Sunday..
The traditional story of Mary's birth is, of course, completely apocryphal. The earliest account comes from the very influential (2nd-century) Protoevangelium of James, from which we get the traditional names, Joachim and Anne, for Mary's parents. A different (but obviously not completely unrelated account) also occurs in the Qur'an. Islam considers both Jesus and Mary as having been born untouched by Satan.
Mary's freedom from all sin from the very beginning of her existence is, of course, the Catholic dogma of the Immaculate Conception, which is celebrated on its own feast on December 8. (That too will fall on a Sunday this year. Instead of being completely ignored, it at least will get transferred to the following day. Since, however, the obligation to assist at Mass does not get transferred with it, it too will likely pass largely unnoticed by many.)
Mary's entrance into this world free from original sin is what makes her an exception to the general rule that it is a saint's death but not his or her birth that is celebrated in the liturgy. (According to the traditional interpretation of Luke 1:15, John the Baptist was purified from original sin while still in his mother's womb. Hence his birth is also celebrated - six months before Christmas, on June 24.)
It is generally thought that the current celebration of Mary's birth on September 8 is derived from the 5th-century dedication of a church in Jerusalem on the supposed site of her parents home by the biblical pool of Bethsaida or Bethesda. The crusader-era Church of Saint Anne stands on that site today (photo).
Given the antiquity of the (albeit apocryphal) traditions associated with Mary's birth and the ubiquity of its celebration both Eastern and Western liturgical traditions, we would do well to find some way to revive the popularity of this celebration.