Here's the history behind this oldest of Marian feast days from Encyclopedia of Catholic Devotions and Practices by Ann Ball.
August 15 is a solemnity commemorating Mary being taken into heaven, body and soul, at the end of her life on earth. A truth of faith, it was proclaimed infallibly as dogma by Pope Pius XII in 1950.
By the fifth century, a feast called the Memorial of Mary was already being celebrated on August 15 in the Eastern Church. Over time it came to be known as the “falling asleep,” or, in Latin, the dormitio. Early Christians believed Mary “fell asleep” at death and rested until she awoke in heaven.
Emperor Mauricius Flavius (r. 582-602) decreed the Feast of the Dormition would be celebrated on August 15 throughout the Byzantine Empire. A basilica in Gethsemane marked the spot where, according to popular belief, she had died. Today in Eastern liturgies, the feast is more commonly called the Assumption or “Journey of the Blessed Mother of God into Heaven.”
Rome adopted the feast in the seventh century and its title became Assumption.
In the West, from the very beginning, it focused on Mary’s bodily assumption into heaven. During the sixteenth century, the Assumption became the greatest of the Marian liturgical celebrations and one of the most prominent of the Church year.
In Hungary, there is a tradition that the first Hungarian king, St. Stephen (d. 1038), offered his royal crown to Mary and made her the patroness of the country. Pageants, parades, and rejoicing marked their annual celebration of the Assumption. Delegates from all parts of the country brought gifts from their harvest to Budapest. In Poland, a similar presentation included bringing wieniec, or harvest wreaths, to the president in Warsaw. A traditional play was performed in France where a flowery platform with figures of angels was lowered within the church to a flower-covered sepulchre and was raised with a statue of the Virgin while boys dressed as angels played and the people sang Marian hymns. In Austria, the people processed through the fields led by the priest who asked God’s blessing on the harvest. Processions in honor of the day were held in most of central Europe, France, Spain, Italy, and in South America.
In the Italian procession, a statue of Mary was carried through the streets of the town until it was met by a statue of Christ. Three times, the images were inclined toward each other as if bowing, then the Christ conducted his mother back to the parish church for benediction. The procession is called Candelieri in Sardinia because huge candles of wax are carried through the streets to her shrine in fulfillment of a promise made in 1580 when a deadly plague was stopped on August 15 after prayers to the Virgin.
Herbs picked in August were said to have particular healing power. In central Europe during the Middle Ages, the Church elevated a popular belief of pre-Christian times and made it a Christian rite with deep meaning by holding the “Blessing of Herbs” on Assumption Day. This practice remained in effect until recent reforms. The Eastern Rites had similar blessings. The Syrians even celebrated a special Feast of Our Lady of Herbs on Mary 15. The Armenians brought the first grapes to church on Assumption Day to have them blessed. The Sicilians kept a partial abstinence from fruit during the first two weeks of August and on the feast day had all kinds of fruit blessed in church to be served at dinner.
Baskets of fruit were a customary gift for the Feast of the Assumption. In the German sections of Europe, the time from August 15 to September 15 was called “Our Lady’s Thirty Days,” and many of the medieval shrines show the statue of Mary clad in a robe covered with grain. A robe of grain is still a popular feature of Assumption decorations in many places.
Another old and inspiring custom on the Assumption was the blessing of the elements of nature as the source of human food.
In many parts of Europe, the priests blessed the fields, orchards, and farms on that day. In the French Alps, the priest rode from pasture to pasture with an acolyte sitting behind him holding the holy water. At each meadow, the priest blessed the animals which were gathered around a large cross decorated with flowers. In the Latin countries, especially Portugal, the ocean and the boats of the fishermen were blessed. This custom is popular today in the United States in a number of coastal towns.