We celebrate The Assumption because that is when Mary went in to dormition (deep sleep) and her body went to heaven. Pope Pius declared that it will be true that Mary’s body went to heaven when Mary went to heaven. Mary was the Queen of the Heaven Her Heavenly birthday was the 15th of August.
Pope Pius was the one to tell everyone that it is true that Mary’s soul and her body went to heaven. Mary is the queen of heaven.