Why do we Romans celebrate such difficult feasts?
How many rush to Mass on December 8th because they are dying to celebrate this feast as opposed to the number who come because of obligation - not to mention the number who do not come at all because, in ignorance or confusion, they cannot see a reason to? If Pius IX declared this feast to bring us together, it does not seem like he accomplished that goal.
And to add to the confusion, it is not a holy day of obligation throughout the Church, especially if it falls on a Sunday and gets moved.
Additional confusion comes from the name: it is Mary's conception - not Jesus' -- that one is celebrated on the Feast of the Annunciation.
And then there is the "why?" Why declare this feast? Why declare the doctrine?
While theologically and devotionally Mary has been considered the greatest and holiest of the saints by the Church Fathers and Doctors and that the greatest amount of honor (hyper-dulia) was due her, it took a long time for this doctrine to develop. Most had difficulty in seeing the human Mary as sinless — whether at her conception or at any other time of her life. Would not that kind of thinking take her out of reach of the everyday faithful? Why would God want to do that when everything else He does is to help us? Even the likes of the Mary bandwagon drivers Thomas Aquinas and Bernard of Clairvaux could not completely justify a theological reason for this teaching. So, the secret? This is one of the Church teachings that arose from a grass-roots devotion; it developed more from the piety of the faithful than from the insights of brilliant theologians. So, when Pius declared it, it was something that bound the faithful together.
Today though, there does not seem to be much grass-roots support for this feast; it feels more flipped, like a theological movement rather than a grass-roots one. Probably because of the theological attempts to make sense of it. The logic of an at-birth sinless Mary is difficult to explain, though even if the reasons can be logically accepted.
So why keep at it? First off, we Catholics should not be afraid of a bit of Mystery. We are surrounded by Mysteries, we embrace them. Perhaps the how and why of Mary being "full of grace" are not as necessary to understand as the fact that we have all been given this same grace through the Death and Resurrection of Christ - and that, just maybe, she was given this grace early in order for her to be truly free in order to say "yes", as we are now truly free to say "yes". That is what we can celebrate on this or any difficult feast, and why we should flock to Mass to do so.
When Elizabeth heard Mary’s greeting, the infant leaped in her womb, and Elizabeth, filled with the holy Spirit, cried out in a loud voice and said, “Most blessed are you among women, and blessed is the fruit of your womb. And how does this happen to me, that the mother of my Lord should come to me? For at the moment the sound of your greeting reached my ears, the infant in my womb leaped for joy. Blessed are you who believed that what was spoken to you by the Lord would be fulfilled.”
-- Luke 1:41-45