In my humble opinion, Brittany is one of the loveliest areas in France. I offer as evidence my only picture of Locranon, a town with bodacious hydrangeas.
The Breton peninsula is particularly Celtic, being the land of the Veneti, Osismi, Coriosolites, and other tribes of Caesar's day and before, and being repopulated in the 5th century or so by people from Cornwall.
Brittany has a lot of Catholic processions and fests, called Pardons, but one--the Tromenie--dates back to the Celtic times before Caesar. The Tromenie takes place on the second Sunday of every July, outside the village of Locranon--a place which you may have seen in A Very Long Engagement. The idea is to walk the route of St. Ronan, who founded Locranon.
Pilgrims walk to a little chapel where St. Ronan's supposedly lived and have an open-air Mass.
That's the Petitie Tromenie. Here's a pretty comprehensive site with its history, some music, and lore. It's documented to have gone on since the 11th century, and since St. Ronan lived several centuries before that, my guess is that the practice is much older.
Every six years, though, folks make the Grande Tromenie--a map of that route is on the site, at the bottom of the page. The next Grande Tromenie will be in 2013. These 19th-century postcards show the area--it hasn't changed all that much.
The Grande Tromenie route follows a much longer path from the village well to a clearing called Le Nemeton. That word--Nemeton--is Celtic for temple (that's pretty well documented, even though a lot of the Gaulish language is lost). Twelve markers are passed during the seven and a half mile circuit, representing the twelve months of a lunar calendar. Or, if you prefer the Catholic version, the markers are twelve stations of the cross. It's anyone's guess whether a sacred walk was made yearly during the B.C. years, but I wouldn't bet against it. I was told that some of the markers date to pre-Roman times.
When I went looking for links, I was surprised to find this 1959 article from Time Magazine, describing the Tromenie. An abbreviated version of the legend of St. Ronan and how the walk started, and what the author saw in 1959, is very interesting.