Popular Catholic blogger, Elizabeth Scalia, says she loves the Day: The Immaculate Conception, and gives us 3 reasons why she does

If you ask Catholics which Marian feast day is their favorite, many will tell you it’s this one, and I would be among their number, for three reasons:
I love the day because it arrives just as we are beginning to deepen our experience of Advent. We place ourselves in the desert and give ourselves over to actively feeling and expressing the most ancient and fundamental of our myriad yearnings.
To that end, we light a candle. What a beautiful gesture of hope: here I am, Lord, in this place of emptiness — of hidden dangers and sudden threats, and of weirdly disorienting sameness. I strike a match and hope that amid the glory of your brilliant, star-laden sky, and the rocks and seas which reflect it all back to heaven, you might notice my tiny flame and find me, and come to me, and — if you will not scoop me up to be with you — consent to remain here, with me.
So much of our seeking God is really about our own efforts to be found after having gotten ourselves good and lost. Sometimes this happens when we have been defiantly reckless in our choices, but not always. Our sense of displacement doesn’t necessarily mean we have intentionally stepped where we shouldn’t; it can mean we have simply become so enmeshed within our immediate concerns that we have been neglectful of our place in eternity. When we finally take a moment to look up, we realize night is upon us, and we are far from home. Here in Advent, seeking to be found, this feast is deep, calling on deep.
I love the day because the whole concept of the Immaculate Conception — often misunderstood, even by many Catholics — is a beautiful mystery, one that invites us to consider that a plan has been in effect since ancient days. This is a reassuring reminder, particularly when times (whether in the world or in our personal lives) seem given over to randomness and chaos.
…it is natural to think that Christ’s birth is a reflection of the whole “big bang” of Creation – the first one made the world and everything in it, the second returned it to its origins, wedded in covenant. . .So, if we are thinking along these lines – of creation, intention, something from nothing, voids and space – I’m describing Mary’s virginal womb at the time of the Annunciation. But perhaps I am also describing the condition of Mary’s own soul at the time of her own conception. The God of No Accidents, Who Is, Was, Ever Shall Be, the God who breathed forth Creation with it’s Incarnational mirror already in place, knew his Vessel. He knew from where the Ark of his New Covenant would come, for he would provide it, and in the necessary [pristine] condition. [...] Then there is movement. God stirs, but this time, instead of saying, “Let there be…(light, etc)” it is the created creature who says the Word as she utters her Fiat. “Yes,” she says, “let it be…”
I love the day because it reminds me — as if all that doesn’t make it plain enough — that we are none of us without worth or meaning, that even the least of us are called to play unexpectedly important roles within The Great Plan. Who would have seemed less “seen”, less worthy of notice, less important to the great scheme of things than a 14 year-old Jewish virgin of living among the mean hills of Nazareth? How about a barely-schooled, under-nourished and sickly 14 year-old living in one-room squalor, in the French Pyrenees? The first, confronted with a mystery beyond her comprehension, nevertheless blossoms into a prayer of prophetic praise; the second, confronted by heaven, and words she does not even understand, holds fast and never backs away.