Ineffabilis Deus is an Apostolic Constitution from Pope Pius IX. It is one of the two times the Pope ever has spoken ex catherda and officially enshrined dogma in Church history. This is a Papal Bull about Mary the Virgin Mother of Jesus and her own birth, although many people think that this feast day is about Jesus. In the Papal bull, Pope Pius says “We declare, pronounce and define that the doctrine which holds that the Blessed Virgin Mary, at the first instant of her conception, by a singular privilege and grace of the Omnipotent God, in virtue of the merits of Jesus Christ, the Saviour of mankind, was preserved immaculate from all stain of original sin, has been revealed by God, and therefore should firmly and constantly be believed by all the faithful.”
This is an interesting document as this idea had been around Christianity since the early days with Augustine and Irenaeus talking about it back in the day. In the Middle Ages this idea that Mary was free of sin was disputed by Thomas Aquinas and Bonaventure as how could Mary be free from sin and be redeemed by Jesus. It wasn’t until Franciscan Friar John Duns Scotus who leaning on the Early Church Father brought the idea back out into the light, Mary needed redemption like all human being but through the merits of the Crucifixion given early she was conceived without the stain of Original sin, however the Dominicans didn’t really like the reasoning following the Aquinas logic. This went on for some time and although the idea was taught in school it never was made doctrine of the Church.
Today is the Patronal Feast Day for the United States, Spain, Ireland, Brazil, Korea, Nicaragua, the Philippines and Portugal. So it is a Holy Day of Obligation, if you have some time get to Mass or if you can’t make it perhaps say a Rosary today.