Our mother Mary was, herself, born without sin. That’s why this feast day is about Mary’s conception, not that of Jesus.
One explanation that I’ve heard of this is that the Ark of the Covenant must be unblemished. And since Mary is the Ark of the New Covenant, Jesus, it is appropriate that she also be unblemished.
So it’s about faith, not scientific proof. And I’m quite fine with that.
Because anything which helps bring me closer to God and Jesus is a Very Good Thing.
I’ve grown to have great appreciation for Tradition – because we’re always standing on the shoulders of those giants who have gone before us. So the billions of Catholics who have gone before me believing in the Immaculate Conception help propel my faith forward as well.
I find it interesting that none of the 20 mysteries of the rosary include Mary’s conception. But then I remember that everything points toward Jesus.
That’s what my faith is built on.