Today is a holy day of obligation in the United States, so if you're in the United States, be sure to go to Mass.
Also as a result of it being a holy day, I won't be blogging, but feel to have fun with the archives and the comboxes!
Here's what the Compendium has to say about the Immaculate Conception:
96. What does the “Immaculate Conception” mean?
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.
And here's what the Catechism has to say:
491 Through the centuries the Church has become ever more aware that Mary, "full of grace" through God, was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854:
The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Saviour of the human race, preserved immune from all stain of original sin.
492 The "splendour of an entirely unique holiness" by which Mary is "enriched from the first instant of her conception" comes wholly from Christ: she is "redeemed, in a more exalted fashion, by reason of the merits of her Son". The Father blessed Mary more than any other created person "in Christ with every spiritual blessing in the heavenly places" and chose her "in Christ before the foundation of the world, to be holy and blameless before him in love".
493 The Fathers of the Eastern tradition call the Mother of God "the All-Holy" (Panagia), and celebrate her as "free from any stain of sin, as though fashioned by the Holy Spirit and formed as a new creature". By the grace of God Mary remained free of every personal sin her whole life long.