Today, Catholics around the world celebrate the dogma that at the end of her earthly life Mary was assumed body and soul into heaven.
Today is the great feast of the Assumption of the Blessed Virgin Mary. Catholics around the world celebrate the fact that the Blessed Virgin Mary, “having completed the course of her earthly life, was assumed body and soul into heavenly glory.” (Munificentissimus Deus, 44-45)
The fact that Mary is in heaven with both her soul and body is signficant since, ordinarily, the faithful who die enter heaven with just their souls, awaiting the resurrection of their bodies which will occur in the Last Days.
The feast of the Assumption is a Holy Day of Obligation, which means that Catholics must attend Mass as well as “abstain from those works and affairs which hinder the worship to be rendered to God, the joy proper to the Lord’s day, or the suitable relaxation of mind and body.” (CIC 1247)
Belief in the Assumption of Mary dates back to the early Church and has been held as certain for many centuries. The feast of the Assumption (and it’s close cousin, the feast of the Dormition) has been celebrated on August 15th since at least the middle of the first millenium.
In one of his famous sermons on the feast of the Assumption in the 8th century, St John Damascene preached, “Today the spotless Virgin, untouched by earthly affections, and all heavenly in her thoughts, was not dissolved in earth, but truly entering heaven, dwells in the heavenly tabernacles. […] Today the life-giving treasury and abyss of charity (I know not how to trust my lips to speak of it) is hidden in immortal death.” (Sermon II on the Assumption)
Nevertheless, the doctrine wasn’t elevated to the level of dogma until fairly recently: Pope Pius XII infallibly confirmed the doctrine in 1950 in his Apostolic Constitution Munificentissimus Deus.
A dogma is a teaching that has been proposed infallibly by the Church’s magisterium as something that has been revealed by God. As such, the faithful are obligated to believe it. The denial of a dogma is called heresy, and the obstinate denial of a dogma by a baptized person makes that person a heretic and endangers their soul.
The Assumption of Mary is not to be confused with the Ascension of Christ. Both doctrines mean that the individual is now in heaven with both their body and soul. However, Jesus is said to have “ascended” on his own power, whereas Mary “was assumed” by God and not on her own power.
Here is the section of Munificentissimus Deus in which the Assumption is declared dogma, with the dogma in bold:
(Munificentissimus Deus, 44-45)