I had no idea

so I had to look it up

Apparently:

The Limousin ostensions are a religious and popular tradition dating back to the end of the 10th century. They take place in twenty communes including Limoges and other localities, fifteen of which are in the Haute-Vienne, but also in Charente, Creuse and Vienne. They are held every seven years, with the last edition in 2016.

The Limousin septennial ostensions are now on UNESCO's Representative List of the Intangible Cultural Heritage of Humanity, after this ritual practice was inscribed on the Inventory of Intangible Cultural Heritage in France.

The legend fixes the origin of this religious festival to the year 994, when Limousin, like a great part of Aquitaine, was in the grips of the "mal des ardents", or ergotism, an epidemic which starts at the end of the harvest. This intoxication is caused by the consumption of rye bread contaminated by a parasitic fungus, the ergot of rye. It causes a sensation of excruciating burning and hallucination (hence the name "ardent", from the Latin ardere, to burn), seizures and painful spasms, diarrhea, paresthesias, itching, headaches, nausea and vomiting. Patients also have hallucinations similar in every way to those triggered by LSD, and psychiatric disorders such as mania or psychosis.

In 994, this disease was seen as a punishment from God. In Limoges, the sick, who had come to implore divine protection, crowded into the churches. Faced with the extent of the tragedy, Bishop Hilduin and his brother Geoffroy, abbot of Saint-Martial, decided to organize a large gathering around the relics of several Limousin saints.

Ambassadors were sent throughout Aquitaine to invite the archbishops of Bordeaux and Bourges, the bishops of Clermont, Le Puy, Saintes, Périgueux, Angoulême and Poitiers, to meet in council in Limoges. On November 12, 994, after three days of prayer and fasting, the body of Saint Martial, the first of the bishops of Limoges and protector of the city, was lifted from his tomb, placed in a golden shrine, and carried in a procession from the Basilica of the Savior (today's Place de la République) to Mount Jovis (montis Gaudii), outside the walls. This hill bears this name which means Mount of Joy since that time. Today, it is located in the city of Limoges, in the Montjovis district.

The procession was led by all the prelates, the monks of the abbey of Saint-Martial, and William IV, Duke of Aquitaine, followed by many pilgrims. A huge crowd crowded along the route, gradually joined by groups of monks carrying relics from Figeac, Chambon, Salagnac, and many other parishes. Arrived on the hill dominating the city, the relics of the Limousin saints are offered to the veneration of the population in distress. This mass event was the very first ostension (a term that originates from the Latin verb ostendere, which means to show, or expose, and which was first used by Bernard Itier, monk librarian of Saint-Martial Abbey, in 1211). On December 4, when the body of Saint Martial was brought back to his tomb, the epidemic had stopped. The chronicles of the time mention more than seven thousand cures.

Nowadays, the Limousin Ostensions are events of patrimonial, social and tourist interest, which, as the interest that the Church continues to carry to it, keep a strong religious dimension. The organization of these events mobilizes the public actors, the associative world and many private individuals. The last septennial ostensions, the 72nd of the name, took place from February to October 2016. The next ones will take place in 2023.

The septennial Ostensions begin in Limoges, with the "ceremony of recognition of the chiefs ". The shrine of Saint Martial is opened jointly by the mayor of the city, the bishop, the parish priest of Saint-Michel-des-Lions and the first bayle of the great brotherhood of Saint Martial. In a second time, the heads of Saint Loup and Saint Aurélien are taken out of their shrine. A torchlight procession then stretches to the cathedral of Saint Stephen.

The following Sunday, the Primate of Gaules and Archbishop of Lyon celebrated an opening mass. The Limousin ostensions are framed by the raising and lowering of the amaranth and white flag at the top of the church of Saint-Michel-des-Lions.

Translates by DeepL from https://fr.wikipedia.org/wiki/Ostensions_limousines