Much of the knowledge which The Church has about the details of the life of the Theotokos comes from Holy Tradition, in other words, that body of information held by the early Church against which the bishops measured the canonicity of various spiritual writings of the era to determine what would make up the canon of the NT. There are also a number of apocryphal writings, like the Protoevangelium of St. James which describe events in her life in great detail. The celebration of the birth of the Theotokos as a feast day has been going on in The Church from the 1st Century, at least in the East.
Thank-you. It has to be from Tradition because it's not straight out of the Bible. I didn't think that there were such distinct dates for Mary in Traditional writings. It is interesting to read about these, but not so easy to believe for someone not in the Orthodox Church, IMHO. I will seek out some more information on this from the internet.