What explains the next thirty-three years?
Rather more than thirty three. Tradition says that Mary lived on following Jesus' Resurrection and Ascension.
The Church Fathers and Doctors of the Church write that Mary's perpetual virginity is a witness to the splendor and uniqueness of the Incarnation. As Mary herself says in Luke's Gospel...."henceforth all generations will call me blessed". We believe that the Incarnation was an event unprecedented in history, before or since and that God witnessed to this fact and confirmed it by Mary's perpetual virginity.