I did not know this existed. It is in Paris. Google translation from the website of Fr Pivert: https://abbe-pivert.com/ostension-de-la-sainte-tunique-dargenteuil/


Published on 20 April 2025 by M. l'Abbé Pivert

Exposition of the Holy Tunic of Argenteuil

An exceptional exposition of the Saintes Tunique d'Argenteuil takes place in the Basilica of Argenteuil until 11 May, every day from 10 a.m. to 10 p.m. except Monday, 28 April and 5 May. You will, of course, not participate in the ceremonies organized by the diocese.
I strongly encourage you to worship this sacred relic. We'll go there in community, I'll keep you informed so that you can join us.

Practical information
The Holy Tunic, Relic of the Passion of Christ
The Holy Tunic of Argenteuil is the garment worn by the Christ of the Holy Supper at the foot of the Cross. It is an exceptional witness to the Passion: the piece of wool is stained with the blood of Jesus. This garment has survived and passed on to us for almost 2000 years. This garment worn by Christ is mentioned in the Gospels (John 19:23-24) as the garment drawn by lot by the Roman soldiers after the crucifixion.
There has been much evidence of authenticity from scientific studies over time, including analysis of pollen, weaving and blood group.
The Jews of the first century wore under the mantle (simba) two shirts: a thick top (cheoneh), one light underneath (sadin). The tunic (Greek khiton) was fitted to the body and had sleeves. It was sometimes carried directly on the body, sometimes on a very large and very long wool shirt.
At the time, this coat was an underwear, and it was necessary to wear a second tunic over it. In natural wool dyed with garance, it was very simple in terms. Wool has not been mixed with any plant substance.
The Sainte Tunique d'Argenteuil is a woven garment, which means that it was made on a loom. It is unsuitable, that is, “seamless”. It was made of one piece, without any different pieces of fabrics being reunited.
Probably redeemed from the soldier by the disciples, the Tunic had to become very quickly for the nascent Church a valuable testimony of the Passion of Christ. The first centuries of the history of the Tunic remain in part hidden. Because of the Persian and Muslim invasions in the ^7th century, the relics then in the Holy Land were collected in Constantinople and the empire of the East, weakened, sought to the West of the Allies. At the end of the ^8th century, Empress Irene, sent diplomats to the new Western emperor, Charlemagne.
She joins its embassy prizes: among them, the Tunic of Christ. The very Christian Charlemagne entrusted the relic to his daughter Theodrade then nun at the monastery of the Humility of the Incarnation in Argenteuil. The Tunic reportedly arrived there on 12 August 800 at 1 p.m. The Norman invasions of the ^9th century drove nuns out of their monastery. They hide the Tunic in the walls of the monastery and it was only two centuries later that the monks of Saint Dinys, undertaking works at the monastery, found it nestled in a wall.
It was in 1156 that the first ostension took place, organized by Hughes d'Amiens, Archbishop of Rouen, in the presence of King Louis VII, many bishops and a great crowd of worshippers. On that occasion, the charter of 1156, the record of the ions, was drawn up.
During the Hundred Years' War, the abbey was again plundered and burned in 1411 and the parish church was only rebuilt in 1449. The "Dream of God" is then the object of many pilgrimages, notably the kings of France Francis I^er, Henry III, Louis XIII, the queens Marie de' Medici and Anne of Austria, but also of Cardinal Richelieu. Large processions took place, a sign of great popular piety also giving rise to many miracles.
During the Wars of Religion in 1567, the Tunic was saved when Argenteuil was taken by the Huguenots.
During the Revolution, the Benedictine priory was suppressed and the relic returned to the parish church. On 18 November 1793, in the face of the threat of confiscation of the property of the Church, the priest of Argenteuil Ozet (1749-1816) cut it into pieces and entrusted some to parishioners. He buried large pieces in his garden before being imprisoned for a year.
In 1795, released, he found the Tunic, but failed to gather all the pieces.
Pilgrimages and solemn ostensions resumed in the ^19th century. In 1866 the new church was built. In 1894, a solemn ostension took place after the Bishop of Versailles commissioned scientific studies on textiles. These same studies identify human blood on the Tunic.
For this ostension, each of the 20 existing pieces of the Tunic are attached to a tunic-shaped satin support fabric. It is this shape that is known to this day. Then the pieces were fixed on a brown support which better restores the appearance.

I know this is Wikipedia, but it seems there are a few different traditions regarding this relic.  How are we to know which one is authentic?

Seamless robe of Jesus - Wikipedia

The Seamless Robe of Jesus (also known as the Holy Robe, Holy Tunic, Holy Coat, Honorable Robe, and Chiton of the Lord) is the robe said to have been worn by Jesus during or shortly before his crucifixion. Competing traditions claim that the robe has been preserved to the present day. One tradition places it in the Cathedral of Trier, another places it in Argenteuil's Basilique Saint-Denys, and several traditions claim that it is now in various Eastern Orthodox churches, notably Svetitskhoveli Cathedral in Mtskheta, Georgia.