The Church has received the tradition of the Nativity (birth) of the Mother of God from the ancient Fathers of the Church.
The Virgin Mary was born to her parents about 16 or 17 years before the birth of Christ. Her father Joachim was descended from the royal line of David, while her mother Anna was descended from the priestly tribe of Levi. They lived faithful lives, but were never granted children, and they were widely ridiculed by their fellow Jews, supposing they were cursed. Joachim and Anna desperately prayed for a child, and promised that they would dedicate it to the Lord.
Their prayer was heard, and Anna conceived a daughter. (The conception is celebrated on December 9.) According to tradition, a daughter was born nine months and one day later. They named her Mary, after Anna's mother. True to their word, Joachim and Anna dedicated their daughter to the Lord when she was three years old, in a feast known as the Entrance of the Theotokos into the Temple.
The feast of the Nativity of the Theotokos is celebrated on September 8.