I will give here what is known about the sash and, as you imply, much of it is shrouded in mystery. It is a mixture of fact and legend, so that in the end we cannot really be certain that the belt ever belonged to Mary.
The famous sash, sometimes called a belt or girdle, is kept in the Cathedral of St Stephen in Prato, near Florence. Each year the sash is shown to the public on five occasions: Christmas, Easter, 1 May, 15 August, and 8 September, the feast of the Birth of Our Lady. On 8 September the relic is carried in procession outside the cathedral for a special blessing of the faithful, given from an outdoor pulpit designed by the famous Renaissance sculptor Donatello (1386-1466). The ceremony, which has been celebrated since the 13th century, is called the “historical procession.”
The story of the relic has its origin in an ancient legend from the fifth or sixth century, which relates how Our Lady, at the moment of her Assumption, entrusted her sash to the apostle St Thomas, who preached the faith in India.
The story has been taken up and embellished by some local histories, consolidated in the 13th century, according to which the sash was given to a priest in Jerusalem and was carefully safeguarded and handed down by his heirs for over a millennium. Only around the year 1141 did the sacred sash come into the possession of a young pilgrim from Prato, a merchant named Michele Dagomari. It was a nuptial gift from a girl from Jerusalem named Maria, whom he married in secret. According to the story, the girl was the daughter of an oriental rite priest who was heir to the relic.
Michele kept the sacred sash in a secret place in Prato and only on his deathbed, around 1172, did he entrust it to the parish church of St Stephen, the current cathedral. Tradition records numerous extraordinary events linked to the relic, with veneration already documented before the middle of the thirteenth century. This led to the sash being immediately considered the most precious treasure of the city. So many people went to see it that its public viewings inside and outside the cathedral were regulated by the statutes of the municipality, which was responsible, as it still is today, for holding one of the keys necessary to remove the relic from its safe location inside the altar.
The relic is kept enshrined in a beautiful altar in a side chapel at the rear of the Cathedral of Prato. Pilgrims come from all over the region and the Catholic world to pray and to honour Our Lady at this sacred site. The relic has been revered at least since the thirteenth century by the people of Florence and the surrounding region in the old Papal States.
This devotion is also known in the East. In the Byzantine tradition there is a feast, celebrated on 31 August, which commemorates the placing of the Cincture of the Most Holy Theotokos, or God-bearer, in the church of the Blessed Virgin in Halkoprateia-Constantinople. It is now venerated in the Vatopedi monastery on Mount Athos. In the Syriac Orthodox Church the sash is known as the Holy Girdle and it is kept in Homs, Syria.
Over the centuries saints and popes have visited the relic in Prato, among them St Francis of Assisi, St Bernard, Pope Pius VII, Pope Pius IX, and Pope St John Paul II. This unique relic is made of ancient goat skin and is light green in colour, a sign of hope and of the life and presence of the Blessed Virgin.
As is clear, the story of the sash is a mixture of history and tradition. In the words of Hillaire Belloc, “legend and myth testify to the truth of tradition.” As with other relics of this kind, the story of the sash is rooted in history, formed by layers and layers of accumulated stories passed on through the centuries, revealing important kernels of historic truth.
The veneration of the sash has been regarded as especially helpful for pregnant women.