The Apostolic constitution, Munificentissimus Deus, was the ex cathedra statement of the dogma of the Assumption of the Blessed Virgin Mary, proclaimed by Pius XII, in 1950. The only other instance, of the exercise, of infallibilty was in 1854, when Pius IX proclaimed the dogma of the Immaculate Conception of the Blessed Virgin Mary. Even these official dogmatic statements were not new. These articles of belief were widely understood since the beginning of christianity. As with the New Testament, and the doctrine of the Trinity, they took time to formulate into declarative statements.
We believe at the end of her earthly life, the Mother of Our Saviour, Mary the Theotokos fell asleep. Later she was bodily assumed to heaven. Before her Dormition, the remaining apostles gathered in the house of Saint John to see her.