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 Immaculate Conception is a Holy Day of Obligation- Vigil Mass on Thursday, Dec. 7th at 4:30 and Friday, Dec. 8th at 7:30 a.m. 11:00 a.m., and 7 p.m.