Eastern and Western Christianity commemorate the same 15 August date but with formally different doctrinal content: Orthodox tradition speaks of the Dormition, affirming that Mary died a genuine, natural death before her bodily assumption into heaven, while the Catholic dogma of the Assumption, formally defined only in 1950, deliberately leaves open whether Mary died before being assumed, a distinction that reflects centuries of theological development proceeding along somewhat different lines in the two traditions even while converging on a shared core conviction that Mary was taken bodily into heaven.
Neither tradition claims direct scriptural warrant for the belief in the way doctrines with explicit New Testament textual support can, and both instead ground the teaching in a combination of ancient liturgical practice, patristic reflection and the theological reasoning that Mary's unique role as the mother of Christ implied a unique destiny for her body as well as her soul, an argument developed at length by theologians across both the Eastern and Western traditions over many centuries.
The feast remains one of the most widely and solemnly observed Marian feasts in both Eastern and Western liturgical calendars, marked in many Orthodox and Catholic countries by public processions, a fast observed in the preceding two weeks within Orthodox practice, and, in several countries, a public holiday reflecting the feast's deep cultural as well as religious significance.