The Annunciation, observed on March 25, celebrates Mary’s miraculous conception of Jesus, who has no human biological father. Because the angel Gabriel “announced” the conception to her (Luke 1:26-38), the feast is known as the Annunciation. Mary’s response to Gabriel, “May it be done to me according to your word” (Luke 1:38b), sets the pattern of openness for everyone who follows Jesus.
The feast of the Immaculate Conception, observed on December 8, celebrates the conception of Mary by her parents (Joachim and Anne, according to an early Christian tradition). The conception was natural except that Mary was preserved from original sin. Even though this teaching was solemnly defined only in 1854 by Pope Pius IX, Catholics had believed it and celebrated it liturgically for many centuries.
Because there is no Gospel story specifically about the conception of Mary, the Catholic Church uses the same passage (Luke 1:26-38, Gabriel’s announcement to Mary) on each solemnity—even though each celebrates a different conception.