Assumption (Dormition) Declared de Fide by Pope Pius XII  in 1950.  This is one of the most ancient of doctrines concerning the BVM, though it was the last to be solemnly declared.  This doctrine has been held as true since early days of the Church.  Catholic Church has relics of nearly any saint, yet there are none of Mary. 

St. John Damascene: "It was fitting that she, who had kept her virginity intact in childbirth, should keep her own body free from all corruption even after death. It was fitting that she, who had carried the Creator as a child at her breast, should dwell in the divine tabernacles. It was fitting that the spouse, whom the Father had taken to himself, should live in the divine mansions. It was fitting that she, who had seen her Son upon the cross and who had thereby received into her heart the sword of sorrow which she had escaped in the act of giving birth to him, should look upon him as he sits with the Father. It was fitting that God's Mother should possess what belongs to her Son, and that she should be honored by every creature as the Mother and as the handmaid of God." 

St. Bonaventure "From this we can see that she is there bodily...her blessedness would not have been complete unless she were there as a person. The soul is not a person, but the soul, joined to the body, is a person. It is manifest that she is there in soul and in body. Otherwise she would not possess her complete beatitude.”