The Immaculate Conception is perhaps one of the most misunderstood doctrines of the Catholic Church. The Immaculate Conception does not refer to the conception of Jesus; rather, it refers to the conception of Mary in the womb of her mother, St. Anne. Another misunderstanding is that Mary did not need a saviour. In the eyes of the Catholic Church, original sin is transmitted to every subsequent generation after Adam and Eve. Original sin can be understood as the hereditary fallen nature and moral corruption that is passed down from Adam to his descendants. Just as we inherit the colour of our eyes and hair from our parents, we also spiritually inherit original sin from them. Everyone is conceived bearing the stain of original sin, and we require baptism in order to make our souls clean. Mary would have also been conceived the same way, however, God gave Mary the singular grace and privilege of the Immaculate Conception to prevent original sin from being transmitted to Jesus.
We declare, pronounce and define that the doctrine which holds that the Blessed Virgin Mary, from the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Saviour of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful 
With these words, Pope Pius IX declared Mary’s Immaculate Conception to be dogma. Pope Pius IX was affirming a belief held by many Christians that came before him, from East and West, that Mary was conceived free of the stain of original sin. God chose to intervene and from the first instant of her existence she was in the state of sanctifying grace and was free from the corrupt nature original sin brings. By the Immaculate Conception, Mary is saved before the Passion, Death, and Resurrection in the same way that the rest of humanity is saved in the Passion, Death, and Resurrection. That is, the salvation of Mary anticipates the salvation of humanity.
The Feast of the Assumption or the Feast of the Dormition (falling asleep) of Mary is also a dogma of the Catholic Church. Belief in the Assumption has been part of the Church’s faith and teaching since the earliest centuries: the Patristic period. The teaching is appropriately summarized in the Catechism of the Catholic Church: “The Most Blessed Virgin Mary, when the course of her earthly life was completed, was taken up [“assumed”] body and soul into the glory of heaven, where she already shares in the glory of her Son’s Resurrection, anticipating the resurrection of all members of His Body”  . This teaching was infallibly defined as a dogma of the Catholic Church by Pope Pius XII in the apostolic constitution entitled Munificentissimus Deus (1950). Pope Pius XII solemnly defined the dogma of the Assumption as follows: “The Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory”  In defining the Assumption as a revealed dogma, Pope Pius XII left open the question of whether Mary “died.” The definition intentionally uses the ambiguous phrase, “having completed the course of her earthly life.”
There is a strong theological connection between the Assumption and the Immaculate Conception, in the sense that both are special favours granted to Blessed Mother as the prefigure of the Church.
These two privileges are most closely bound to one another,” he continues. Christ overcame sin and death by his own death, and one who through Baptism has been born again in a supernatural way has conquered sin and death through the same Christ. Yet, according to the general rule, God does not will to grant to the just the full effect of the victory over death until the end of time has come. And so it is that the bodies of even the just are corrupted after death, and only on the last day will they be joined, each to its own glorious soul. 
Pope Pius XII goes on to say: “Now God has willed that the Blessed Virgin Mary should be exempted from this general rule. She, by an entirely unique privilege, completely overcame sin by her Immaculate Conception, and as a result she was not subject to the law of remaining in the corruption of the grave, and she did not have to wait until the end of time for the redemption of her body.”  Since Mary was preserved from original sin in her Immaculate Conception, and since she sustained the fullness of grace given to her by God (Luke 1:28), Our Lady could not have experienced the consequences of original sin. So Mary also triumphed over the bodily corruption of death in her glorious Assumption.
The Marian dogmas of the Immaculate Conception and the Assumption are not easy beliefs to grasp, especially for non-Catholics. With the Immaculate Conception, it may seem like Mary did not need Jesus to be her Saviour; however, we must remember that it was by Jesus and for Jesus that Mary was created sinless. Without His grace, it would be impossible for such a thing to happen. Jesus is without sin because He is God; Mary is without sin because Jesus made her so. Similarly, the Assumption of Mary is a point of confusion as the Church has never formally defined whether she died or not. The dogma teaches that Mary as a whole person was taken into heaven after the course of her earthly life. It does not specify where, when or how her life ended. In fact “at the end of her earthly course” does not explicitly state that she “died” in the normal sense. The dogma is therefore non- committal concerning her death, and Vatican II adopted the same position.  Both opinions are acceptable and accepted: 1) The immortality of Mary is derived from the meaning of Mary’s privilege of the Immaculate Conception. She is sinless thus exempted from certain effects of sin. The exemption from death would be logical; 2) the death of Mary seems equally logical for those who insist on her perfect conformity to Christ. Mary was Christ’s first disciple and perfect associate. She participated in a real but subordinate role in his saving work. Her participation in Christ’s death would also be logical. We speak of Mary’s “Assumption”, not her “Ascension”. Christ ascended, but Mary was assumed into heaven. In other words, unlike her Son Jesus, Mary didn’t “go up on her own power” to heaven, but was taken up by the power of God. The Assumption, then, is something God did “for” her, like her Immaculate Conception.