The Immaculate Conception, according to the teaching of the Catholic Church, was the conception of the Blessed Virgin Mary in the womb of her mother, Saint Anne. Mary was conceived naturally but God kept her soul immaculate, i.e. free from original sin. The official confirmation of the dogma was defined in 1854 by Pope Pius IX, although the belief that Mary was sinless and conceived free from sin is very old.
The Feast of Immaculate Conception is celebrated in Catholic Church on December 8. In many countries it is a holy day of obligation, among others, in the United States, Argentina, Austria, Italy, Portugal and Spain.
Immaculate Conception is sometimes confused with the conception of Jesus Christ in Mary's womb, while it refers to Mary herself.
Is the Feast of Immaculate Conception Obligatory in 2013?
No, at least in the United States, the obligation to attend Mass is dispensed because December 8 falls on Sunday and the celebration is transferred to Monday, December 9. The transfer is made as the Second Sunday of Advent takes precedence.