On 1 November 1950, His Holiness Pope Pius XII solemnly defined the dogma of the Assumption of the Blessed Virgin Mary in his Apostolic Constitution Munificentissimus Deus. If you haven’t already read it, have a look at the whole document. It’s beautiful.
Here’s an excerpt:
“…after we have poured forth prayers of supplication again and again to God, and have invoked the light of the Spirit of Truth, for the glory of Almighty God who has lavished his special affection upon the Virgin Mary, for the honor of her Son, the immortal King of the Ages and the Victor over sin and death, for the increase of the glory of that same august Mother, and for the joy and exultation of the entire Church; by the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma:
that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.
Hence if anyone, which God forbid, should dare willfully to deny or to call into doubt that which we have defined, let him know that he has fallen away completely from the divine and Catholic Faith.”
O God, who hast taken to thyself the Blessed Virgin Mary, Mother of thine Incarnate Son: Grant that we, who have been redeemed by his blood, may share with her the glory of thine eternal kingdom; through the same thy Son Jesus Christ our Lord, who liveth and reigneth with thee in the unity of the Holy Spirit, one God, now and for ever. Amen.