The Immaculate Conception is sometimes confused with the virgin birth, but it concerns Mary's own conception — that from the first moment of her existence in the womb of her mother Anne, she was, by a singular grace, preserved free from the stain of original sin. The doctrine had been held in popular devotion for centuries and was defined formally by Pope Pius IX in 1854.
The theology turns on what it means for God to prepare a worthy dwelling place for his Son. Mary's freedom from sin is not by her own merit but by the merits of Christ applied to her in anticipation — the first fruit of the redemption he would win on the cross. The feast is the patronal solemnity of the United States, and it falls in Advent, a clear light in the season of waiting.