Today is, in the Catholic Church, the feast of the Immaculate Conception.
In case you don’t know what that is – it’s the conception of Mary the Mother of God, who was free of the stain of Original Sin (thus “immaculate”). It does not refer to the virginal conception of Christ – the feast for that is, logically enough, on March 25th, and it’s called the Annunciation.
It’s really rather sad how often people get this wrong.
Anyway, the doctrine of the Immaculate Conception was defined “ex cathedra” by Pope Pius IX in 1854. The feast is thus a specifically Catholic holy-day, and it is in some ways a symbol of the power of the Pope and the authority of the Church.
This authority is one of the reasons I’m Catholic. It makes much more sense to me to have a living Church that has lasted for 2000 years (I think the very fact that the Church has lasted so long is some sort of evidence for its truth) guaranteeing the truth of Scripture, and making clear whatever is ambiguous there, than to have Scripture by itself – and why should I accept Scripture as authoritative if nothing gives it authority, anyway? – and have everyone believing something different about it.
I do sometimes wonder what Protestants make of that argument.