The Immaculate Conception of Mary, the Mother of Jesus, is the belief that Mary was free from sin from the first moment of her being. This feast celebrates God's victory over sin and evil by his fullness of grace in Mary from the very beginning of her existence. In Christ, the new Adam, the original sin of humanity is conquered and salvation celebrated. It is appropriate that Mary, whose cooperation brought Christ into this world, experience God's grace from the outset of her life.

This teaching on Mary's Immaculate Conception was made official in the Church, after worldwide consultation with the bishops, on December 8, 1854 by Pope Pius IX. Even prior to this official proclamation, the bishops of the U.S. in 1846 determined that Mary, under the title of the Immaculate Conception, would be the patroness of the country.

December 8 is a holyday of obligation. When the feast falls on a Saturday or Monday, as it does this year, Catholics still observe it that way because the Immaculate Conception is the patronal feast day of the USA.

Catholics may fulfill the obligation for the feast on Friday evening or any time on Saturday. If they attend Mass on Saturday night for the feast day obligation, they would also need to attend Mass on Sunday for their Sunday obligation. There are two obligation days in succession and attendance at Mass is required for both.