August 15th is the great feast of the Assumption of Mary into heaven, a holy day of obligation for Catholics.
No, the feast is not about us guessing (or “assuming”) that Mary is in heaven; rather, the feast celebrates the dogma of the Catholic faith that Mary, at the end of her earthly life, was taken up body and soul into heaven, where she lives to this day. This is different than what happens to most Christians, whose bodies remain on earth while their souls go to heaven.
But why is this called an “assumption”? Our Lord Jesus also went to heaven, body and soul, at the end of his time on earth, but we refer to it as his “ascension.”
The difference has to do with who Mary and Jesus are and how they got to heaven.
Jesus Christ is the Son of God made man, with both divine and human natures. When he went to heaven, body and soul, at the end of his earthly life, he did so on his own power. Jesus himself was the active agent. Thus, he ascended (active) into heaven.
Mary, on the other hand, is of course merely a creature, dependent entirely on the gratuitous grace of God for anything. She could not and did not go to heaven on her own power, but instead was taken there. Thus, she was assumed (passive) into heaven.
Catholic theology is generally very careful with its words. Often, as in this case, a seemingly small difference in terminology can be a sign of important distinctions.