The “Old” Catholic Encyclopedia (the first volume of which appeared over 100 years ago in 1907) identified the Assumption as Mary’s principal liturgical feast and assigned it “a double object: (1) the happy departure of Mary from this life; (2) the assumption of her body into heaven.” Thus, well before Pope Pius XII’s dogmatic definition of the Assumption in 1950, the Catholic Encyclopedia attested to the antiquity of the traditions which underlie the Church’s belief.
A 10-minute Vatican video of the 1950 dogmatic definition (with the commentary entirely in Italian) can be accessed at https://www.youtube.com/watch?v=4NRtx6mB2pQ. A 30-minute French version can be watched at https://www.youtube.com/watch?v=4cUt-DDNkXw#t=47.500006 and a very brief English-language version at https://www.youtube.com/watch?v=PJcSyaRU0kc
The centerpiece of that grand event was, of course, the solemn papal proclamation of the Apostolic Constitution Munificentissimus Deus.
Ecclesiologically, it is interesting how the Pope framed the process of preparing the definition – in particular the 1946 inquiry sent to the world’s bishops: “Do you, venerable brethren, in your outstanding wisdom and prudence, judge that the bodily Assumption of the Blessed virgin can be proposed and defined as a dogma of faith? Do you, with your clergy and people, desire it?” (MD 11).
The almost unanimous response of the world's bishops affirmatively confirmed the traditional popular belief.
Munificentissimus Deus is also especially interesting for the its appreciation of the role of the Church's liturgy in expressing the Church's faith. On this Assumption Eve, we may also wistfully recall the Pope's reference to how his predecessor Saint Leo IV (Pope 847-855), who built the defensive wall that still encloses much of the Vatican, "saw to it that the feast, which was already being celebrated under the title of the Assumption of the Blessed Mother of God, should be celebrated in an even more solemn way when he ordered a a vigil to be held on the day before it" (MD 19). Today's Vigil of the Assumption survived the “first cut” when various other vigils like that of the Immaculate Conception were axed in 1955. Among the most ancient vigils it manged to survive until the unfortunate final elimination of all vigils by Paul VI in 1969.
(Photo: Oil Painting, Assumption of the Virgin (1515-1518), by Italian Renaissance artist Titian, located at the high altar in the Venetian Basilica di Santa Maria Gloriosa dei Frari)