The birth of Mary was a major turning point in the history of humanity. It marks the time when our enslavement to sin and death was passing away. This little baby girl born to aged parents, Joachim and Anna, began a sequence of historical events that opened the Gates of Paradise, bringing about the union of Heaven and Earth, God and humanity.
Entry of Virgin Mary into the Temple -
November 21
The Entrance of the Theotokos into the Temple, also called The Presentation.
According to Tradition, the Virgin Mary was taken —presented—by her parents Joachim and Anna into the Jewish Temple in Jerusalem as a young girl (3 yrs), where she lived and served as a Temple virgin until her betrothal (~13yrs) to St. Joseph.
"God predestined Her before the ages for the salvation and reclaiming of our kind. She was chosen, not just from the crowd, but from the ranks of the chosen of all ages, renowned for piety and understanding, and for their God-pleasing words and deeds." Saint Gregory Palamas
According to the Gospel of Luke 1:26-38, the Archangel Gabriel appeared to Mary to announce to her that she would conceive and bear a son, even though she "knew no man." According to holy tradition Mary had come home to her parents when she was only fifteen when she was visited by Gabriel.
This feast commemorates the death, resurrection, and glorification of Christ's mother.
According to Orthodox Tradition, Mary died like all humanity, "falling asleep," so to speak, as the name of the feast indicates. She died as all people die, not "voluntarily" as her Son, but by the necessity of her mortal human nature which is indivisibly bound up with the corruption of this world.
To help us in our preparation of this feast, it is preceded by a two week fast.
“The heavens were astonished and stood in awe, and the ends of the earth, O maiden, were sore amazed, for God appeared bodily to mankind as very man. And lo, thy womb hath proved to the vaster and more spacious that heaven’s heights. for this, O Theotokos, the choirs and assemblies of men and angels magnify thy name “
Reference for more on the Life of The Virgin Mary the Theotokos:
Nativity of the Theotokos - 
Entry of Virgin Mary into the Temple - 
Annunciation Of Virgin Mary -
Dormition of the Theotokos - 
Virgin Mary the Theotokos as Mediatress