The following is an excerpt from The Winter Pascha , by Fr. Thomas Hopko
On the ninth of December the Orthodox Church celebrates the feast of the conception of the Virgin Mary by her parents Joachim and Anna. On this major festival which finds its place in the Church's preparation for Christmas, the faithful rejoice in the event by which Mary is conceived in fulfillment of her parents' prayers in order to be formed in the womb, born on the earth, dedicated to the Lord, and nurtured in holiness to become by God's grace the mother of His Son the Messiah.
The Orthodox Church, particularly in the present time, does not call the feast of Mary's beginning the "immaculate conception," although perhaps in ancient times this title would have been fully acceptable. This is not because the Orthodox consider Mary's conception to have been somehow "maculate" or "stained" (macula means "stain" in Latin). It simply means that the Orthodox do not want to support the conviction that God had somehow to intervene at the moment of Mary's conception with a special action to remove the "stain" of the original sin transmitted by the act of human reproduction because, simply put, the Orthodox do not hold that such a "stain" exists.
The Orthodox Church affirms original sin. Orthodox theology teaches that all human beings, including the Virgin Mary who is a "mere human" like the rest of us-- unlike her Son Jesus who is a "real human" but not a "mere human" because He is the incarnate Son and Word of God-- are born into a fallen, death-bound, demon-riddled world whose "form is passing away" (1 Cor 7:31). We are all born mortal and tending toward sin. But we are not born guilty of any personal sin, certainly not one allegedly committed "in Adam." Nor are we born stained because of the manner in which we are conceived by the sexual union of our parents. If sexual union in marriage is in any sense sinful, or the cause in itself of any sinfulness or stain, even in the conditions of the "fallen world," then, as even the rigorous Saint John Chrysostom has taught, God is the sinner because He made us this way, male and female, from the very beginning.
Mary is conceived by her parents as we are all conceived. But in her case it is a pure act of faith and love, in obedience to God's will, as an answer to prayer. In this sense her conception is truly "immaculate." And its fruit is woman who remains forever the most pure Virgin and Mother of God.
Come, let us dance in the spirit!
Let us sing worthy praises to Christ!
Let us celebrate the joy of Joachim and Anna,
The conception of the Mother of our God,
For she is the fruit of the grace of God.
 The feast is officially called The Conception of the Theotokos. Mary's nativity is celebrated on September 8. A popular tradition among the Orthodox says that the nine-month period is purposely off by one day to illustrate the "mere humanity" of Mary, unlike the "divine humanity" of her Son, whose conception on the feast of the Annunciation is celebrated on March 25, exactly nine months before His Nativity.
 See John Chrysostom, On Titus, homily 2.