It is a very ancient festival, even though one cannot say for sure when it was first celebrated. The first mention of it was made in the 4th century. According to Fr. Julian Katriij, the festival was officially established in the Byzantine Church by Caesar Mauritius (582-602). The festival moved west, to Rome, in the seventh century and spread throughout the entire Latin Church.
The Gospel does not tell us about the event on which the festival is based — the birth of the Mother of God. It provides scarce information about the Mother of God: nothing is said about her birth, young years or Assumption. The Gospel does not even mention the names of her parents. All this information is drawn from church tradition and apocrypha. The main source of information on the life of the Mother of God is the Protoevangelium of James written in the second century. We learn from it about the birth of the Holy Virgin Mary to the family of childless Joachim and Anna.
In general, church tradition does not mention the day of birth of saints but instead the day of their deaths as it is the beginning of their heavenly lives. However, there are two exceptions: Mary and John the Baptist. This is because these two persons take special places in salvation, in the work of preparing the way for the advent of the Son of God. Therefore, their births are world-wide events.