The Assumption of the Virgin Mary, August 15. Assumption Day commemorates the belief that when Mary, the mother of JesusChrist, died, her body was not subjected to the usual process of physical decay but was “assumed” into heaven and reunited there with her soul. This holiday, which has been celebrated since the fourth century CE, is a Christianization of an earlier harvest festival and, in many parts of Europe, is known as the Feast of Our Lady of the Harvest.