The commemoration of the death of the Blessed Virgin Mary (the Dormition, or falling asleep, as it was known in the East) is known as the Assumption because of the tradition that her body did not decay but that she was raised up, body and soul, into heaven. This tradition was already present in the sixth century; by the beginning of the twentieth century it was widespread (for details, see this article in the Catholic Encyclopaedia); and after consulting the views of bishops all over the world, the Pope formally and infallibly declared the doctrine of the Assumption to be part of the authentic and ancient doctrine of the universal Church.

White is the colour of heaven. Liturgically, it is used to celebrate feasts of the Lord; Christmas and Easter, the great seasons of the Lord; and the saints. Not that you will always see white in church, because if something more splendid, such as gold, is available, that can and should be used instead. We are, after all, celebrating.