What is meant by the Assumption?
The whole church understands by the term that the blessed virgin Mary, soon after her death and glorious resurrection, was taken up body and soul to Heaven to be forever enthroned above the angels and saints.

Was the Assumption revealed?
Without a divene revelation, the Assumption would not be capable of being defined a dogma of faith, since the motive of faith is the authority of God in revelation.
Hence, that the Assumption should have been known as certain and capable of being proposed to the whole Church for acceptance, a public revelalation must have been made to the Apostles, or at least to one of them- saint John, for example. Note that this revelation must have been made to an apostle since the deposit of common and public revelation was completed with the death of the last apostle[saint John]. It may have been made explicitly or implicitly.
Happy Assumption to all children of Mary