|Theotokos with her parents, Sts. Joachim and Anna.|
Many still think the Immaculate Conception refers to Jesus' conception (which is actually celebrated on March 25th) but this is the day we celebrate the Immaculate Conception of the Blessed Virgin Mary.
Mary, according to the dogmatic definition (no faithful Catholic can deny this) was, from the very moment of her conception, preserved from the stain of Original Sin. The wording from Ineffabilis Deus:
We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in 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 Savior 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.
There is a stern warning to any Catholic who would deny this teaching:
Hence, if anyone shall dare -- which God forbid! -- to think otherwise than as has been defined by us, let him know and understand that he is condemned by his own judgment; that he has suffered shipwreck in the faith; that he has separated from the unity of the Church; and that, furthermore, by his own action he incurs the penalties established by law if he should dare to express in words or writing or by any other outward means the errors he thinks in his heart. (ibid)
Again, this still IS the Second Sunday in Advent - so don't forget to light that second violet candle!
Blessed Advent, my friends!