Adam's sin can be celebrated insofar as it plays a part in a greater narrative that makes the true story of God and Man more profound than it ever could have been without sin. In other words, sin occasions an enrichment of the end of Man.
Meditating on the Passion with St. Alphonsus Liguori
"Jesus on the cross! Behold the proof the love of God; behold the final manifestation of himself, which the Word Incarnate makes upon this earth,--a manifestation of suffering indeed, but, still more, a manifestation of love."
Faith and Reason 