It seems that since 1983 there has been an annual three-day 97km walking pilgrimage from Paris to Chartres. A specifically religious event for supporters of the pre-Vatican II traditional liturgy. About 16,000 walkers accompanied by over 300 priests. I must confess that I had never heard of this particular pilgrimage.
It is not
exclusively for traditionalists, but it is very popular among them.
It's more that the traditionalists and other traditional Catholics have been using it more and more for fostering and rallying support among and between their groups -- but its main focus is still as a students' pilgrimage ; the traditional character of the pilgrimage is really a consequence of students in France who are
actively Catholic tending to be more traditionally so than the average Catholic in the country ; though there is certainly an increasing interest in traditional Catholicism among youth generally.
I myself first heard of this particular pilgrimage in 1993 or 1994, during the training and preparation period for my 2nd 1994 Camino from Paris -- which started through Chartres ; and on that 1994 my feet were just naturally drawn eventually into the route of that pilgrimage, as I could see from a few signs along the way.
In 1997 I did see the departure of one massive group of those pilgrims in Paris, at the time of the World Youth Day in the city that year, and the number and character of them were truly impressive.
As to the record number this year, I guess it's possible that they may need to think about organising a second "overflow" weekend or something ...
---
BTW the young man who drove off the madman in Annecy and likely prevented an even worse outcome is exactly that sort of young student pilgrim, making his way in this case around all of France's Cathedrals ; who dropped his backpack and used his daypack to shield himself and push the attacker away.