The Greatest of the Marian feasts celebrates the Virgin Mary’s participation in her Son’s triumph over sin and death. Not all is finished when we have seen Christ go up to heaven and sit at the right hand of God in order to communicate to us, as the High Priest, the fruits of His sacrifice. The cycle of our redemption can be considered closed only on this glorious day when we see the New Eve join the New Adam in heaven and share with Him the honors of a victory of which she was a chosen instrument... Christ and the Virgin Mary are united in the same work and in the same triumph, in order to restore life to those whom the fault of the first couple had drawn into death” (E. Flicoteaux, O.S.B., “Notre Dame dans l’annee Liturgique”)
It is well to observe at the outset that the cultus paid the Mother of God by the Church existed long before the institution of any feast in her honor. This veneration was expressed by the building of churches dedicated to Mary by Constantine who is said to have built three churches to her in his new capital on the Golden Horn. Grisar (a well-known historian) is of the opinion that there was a church dedicated to the Blessed Virgin in Rome long before the Basilica of Saint Mary Major, generally regarded as the oldest church of our Lady in Rome. This is the church of Santa Maria Antiqua and is regarded by Grisar as the oldest church known to have been dedicated to our Lady not only in the Eternal City, but in the world. There was another ancient church dedicated to our Lady at Ephesus which was know existed at the time of the Council there in 431, and there seems to have been still another at Jerusalem. An itinerary of 570, ascribed to the Holy Land, speaks of the basilica of the Blessed Virgin in the Valley of Gethsemane “in which is shown the tomb from which they say the holy Mary was taken into heaven.”
There is no doubt that the feast of the Assumption itself is Eastern in the origin and that it spread from the East to the West. It first emerged in the clear light of day with the decree of the Emperor Maurice, about the year 600 which ordained that the “Falling Asleep”, or Koimesis, of the Mother of God should be celebrated on the 15th of August. Notice that Nicephorus does not say that the Emperor instituted the feast; he ordered that it should be celebrated on the 15th of August. It would seem that it existed long before that and that he was merely setting a date for its observance. In any case the prestige of his patronage caused the feast and the date to be adopted everywhere in the East. Long before the decree of the Emperor Maurice we can find indications that the feast mentioned so clearly at the time probably existed long before. It is true that it comes into clear light of day for the first time with his proclamation in 600, but this does not exclude the possibility of an earlier observance of a feast comparable to it.
By the ninth century the feast of the Assumption was celebrated everywhere. About 847 Leo IV ordered that it should be celebrated in Rome with an octave and a vigil to be kept by the clergy and the people in the Basilica of Saint Mary Major and that on the Octave Day the station should be celebrated outside the Porta Tiburina in the Basilica Major dedicated to the Blessed Virgin, which had been built by Sixtus III. From the ninth century it has been a holy day of obligation and Boniface VIII gave it the right to be celebrated like the great feasts of Christmas, Easter, and Pentecost even in countries which were under interdict.
By the sixteenth century, the term Dormitio was abandoned altogether and emphasis placed on the Assumption instead. The Assumption had definitely taken its place as the greatest of Mary’s feasts and the solemnity of the fifteenth of August had become one of the greatest feasts of the liturgical year.
Among the Syrians of Antioch and consequently among the Syro - Malankarese, August 15 is also the feast of Our Lady of the Vineyards. The custom blessing herbs on this day is observed until recently in these churches. It cannot be shown to have any definite connection with the feast, although some writers see in it a connection with the ancient legend that the Apostles found nothing in Mary’s grave but freshly blooming flowers.
(Condensed by Fr. Therasian from ‘The history of the feast of Assumption’ by William Shea, The Thomist, Vol XIV (1951), No. 1, Pp.118-133)