Each year on December 8, the Catholic Church celebrates the Solemnity of the Immaculate Conception of the Blessed Virgin Mary, a feast that is often misunderstood. Even though we are preparing to celebrate the birth of Jesus later this month, the "Immaculate Conception" refers to the conception of the Virgin Mary, not of Jesus:

God freely chose Mary from all eternity to be the Mother of his Son. In order to carry out her mission she herself was conceived immaculate. This means that, thanks to the grace of God and in anticipation of the merits of Jesus Christ, Mary was preserved from original sin from the first instant of her conception (Compendium of the Catechism of the Catholic Church 96).

The Church celebrates the birth of Mary on September 8 and celebrates her conception exactly nine months earlier on December 8. Similarly, we celebrate the birth of Jesus on Christmas, December 25, and celebrate his conception exactly nine months earlier on the Solemnity of the Annunciation, March 25.

Some Christians protest against the Catholic dogma of the Immaculate Conception of the Virgin Mary, arguing that Mary would not have needed a Savior if she had no sin. We know that Mary needed a Savior, though, because she herself prayed, “My soul proclaims the greatness of the Lord; my spirit rejoices in God my savior.” (Luke 1:46-47) How can we say that Mary was “saved” if she had no sin to be saved from?

The Catechism of the Catholic Church, quoting the Second Vatican Council, explains that the Virgin Mary is "redeemed, in a more exalted fashion, by reason of the merits of her Son" (CCC 489). Rather than being forgiven for sins, she is preserved from sin entirely, not through her own merits, but in anticipation of the merits of Jesus. A tract from Catholic Answers offers this helpful explanation:

Consider an analogy: Suppose a man falls into a deep pit, and someone reaches down to pull him out. The man has been “saved” from the pit. Now imagine a woman walking along, and she too is about to topple into the pit, but at the very moment that she is to fall in, someone holds her back and prevents her. She too has been saved from the pit, but in an even better way: She was not simply taken out of the pit, she was prevented from getting stained by the mud in the first place. This is the illustration Christians have used for a thousand years to explain how Mary was saved by Christ. By receiving Christ’s grace at her conception, she had his grace applied to her before she was able to become mired in original sin and its stain.

Why was it fitting for God to preserve Mary from all stain of sin? I can think of at least three reasons:

1. Just as sin entered the world through the sin of a woman, Eve, it was appropriate for the Savior to enter the world through the holiness of a woman, Mary. The Catechism, quoting the early bishop St. Irenaeus, says, "The knot of Eve's disobedience was untied by Mary's obedience: what the virgin Eve bound through her disbelief, Mary loosened by her faith." (CCC 494)
2. Because Mary was to bear the Savior of the world within her very womb, it was fitting that God would make her a worthy dwelling-place for his Son by preserving her in holiness. If God insisted that Moses make the Ark of the Covenant out of pure gold, inside and out, in order to carry the Ten Commandments, (Exodus 25:11) doesn't it make sense that he would make Mary pure, body and soul, in order to carry his Son?
3. Finally, keeping Mary free from sin shows us what the salvation of Jesus really looks like. If we are ever tempted to think that avoiding sin is impossible, we can recall that God kept Mary, an ordinary human like you and me, from every sin and can strengthen us in the face of temptation as well. The Immaculate Conception shows that God desires not to overlook our sins, but to radically renew us, conforming our hearts to the Heart of his divine Son. Through Mary's complete holiness, we discover that God offers to save us not by merely tolerating our sins, as we so often seem to think today, but by completely freeing us from them.