This is one of the happiest days in the Church calendar. The day our Spiritual Mother was conceived, protected from all stain of sin by God to prepare her, a fitting vessel to contain the Word of God. Immaculate, and unstained. This allowed her later to say "yes" to the Holy Spirit, receive God, and start us us all on the road to salvation.
When I was in my youth, I did not understand why Catholics teach that Mary had to be sinless in order to bear God's son. After all, a goodly number Protestants do not believe it. But I did not have a proper understanding of the relationship between the Old Testament Ark of the Covenant and the Mary, the New Testament Ark of the Covenant.
Please see this post from my former blog site. http://jmcdhome.wordpress.com/2007/08/15/mary-the-mother-of-god/