Ask many Catholics, and they often get confused on the topic. Commonly, Immaculate Conception is taken to mean the miraculous event of the birth of Jesus.
However, Immaculate Conception refers to Mary, the mother of Jesus. It was about her own conception within her mother’s womb and not when Jesus was conceived. The Immaculate Conception was a gift from God to Mary that enabled her to be able to give Jesus a sinless human nature as His mother. It was always believed as an article of faith by the Church, and only formerly defined as a dogma on December 8, 1854, by Pope Pius IX. Mary was preserved from original sin from the first moment of her conception.
Original sin is a condition inherited through our first parents, Adam and Eve. Their disobedience to God brought about a fallen world, a darkened intellect, a weakened will, and a disordering of the passions. No longer did they have the preternatural gifts of agility, ease of work, health, and continual life. There was an imbalance in nature, and the human race became sick and died. Through ignorance, weakened will, and passions personal sin was introduced. People consciously disobeyed God.
It was the merit of Jesus Christ on the cross that brought about redemption. Through redemption, the soul is elevated by grace, first received in baptism. The world, though saved by Christ, still suffers from concupiscence. However, through grace, humanity can rise above the darkness of sin to the loftiness of God.
Because of the Immaculate Conception, Mary never suffered under the contagion of sin. In God there is no time, but since we exist in time, He prefigured His Son’s redemption and applied it to the womb of Mary’s mother, Ann. Think of an old telephone switch board. The switch operator takes a wire from a call and applies it to an outlet. God anticipates His Son’s salvific work and applies it to Mary. Why is this so important? Because Mary is to give the sacred humanity to the second Person of the Blessed Trinity to redeem us from sin. Therefore, at no time could that humanity be under the contagion of sin itself.
Mary’s sinlessness began from the first moment of her conception. Mary never committed any personal sin from the time she reached the age of reason to the time she died. Since she did not have original sin, she did not deserve to suffer the consequences of it; at the end of her life she was assumed by her son into paradise. The Eastern Church calls this the dormition of Mary. December 8 was chosen for the Feast because nine months later we celebrate Mary’s birthday on September 8.