This day has nothing to do with Mary’s virginity, although I thought it did for most of my Catholic life, but since my Catholic life represents only about a third of my life I didn’t spend too many years in theological error.
Nor does it have anything to do with the “generative activity of her parents,” as the Catholic Encyclopedia rather quaintly puts it; and to be clear, they make sure we know that “her body was formed in the womb of the mother and the father had the usual share in its formation.”
It’s a complicated bit of theology, though, that took the Church centuries to resolve, involving the exact moment when Mary, still in her mother’s womb, became immaculate, i.e. cleansed of original sin, and it all has to do with when the “soul” gets “infused” into the flesh.
And you may think all of this is like arguing about how many angels fit on the head of a pin, until you come up against something like the Stupak amendment.