There were only four occasions for such pilgrimages in the 16th century and then, most likely because of historical events like the 30 Years War and successive smaller wars, the garment was removed from the Trier Cathedral several time to the fortress at Ehrenbreitstein, near Koblenz, to secure its safety and the pilgrimages stopped. When Napoleon attacked the city in 1794, the garment was taken to the interior of Germany for safety and kept at Augsburg. This, however, caused the Bishop of Trier to have difficulty negotiating its return. The garment was restored to the Trier Cathedral in 1810. In celebration of the return, a pilgrimage to the Dom was made by the faithful from many parts of Germany.
In 1891, Pope Leo XIII granted pilgrims who participated in the pilgrimage a plenary indulgence, which brought almost two million people to the Trier Cathedral for the celebration.
The last pilgrimage, held in 1996, was estimated to have brought over a million people to Trier. Since then, annual Holy Robe Days have been held at the Trier Dom. Only during the Holy Robe Days is the Holy Robe chapel of the Dom accessible.
Edgar Christoffel, Der Hochwaldort Zerf am Fuße es Hunsrücks, 1981
Trierer DOM web page - www.dominformation.de
Catholic Encyclopedia - http://www.newadvent.org/cathen/07400b.htm