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. (Since I set half of Death Speaker in Brittany, you may already suspect that I am fond of the place.)
The Breton peninsula is particularly Celtic, being the land of the Veneti, Osismi, Coriosolites, and other tribes , centuries before the Romans arrived. Caesar left it in poor shape, but the area was repopulated in the 5th century or so by people from Cornwall—possibly descendents of the very Veneti, Ossismi, etc. who fled to Britain when Caesar and his legions marched in.
Brittany has a lot of Catholic processions and fests, called Pardons, but one–the Tromenie–may date back to the Celtic times-, way 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 the movie A Very Long Engagement and before that, Tess (Roman Polanski’s version of Tess of the D’Urbervilles).
The idea of the Tromenie is to walk the route of St. Ronan, who founded Locranon over a thousand years ago. During the annual Petite Tromenie, pilgrims walk from this church in Locronan to a little chapel where St. Ronan supposedly lived. Once there, they celebrate an open-air Mass.
That’s the Petitie Tromenie, which has gone on since the 11th century. St. Ronan lived several centuries before that, so my guess is that the practice is much older.
However, every six years, folks make the Grande Tromenie. These 19th-century postcards show the area–it hasn’t changed all that much.
The Grande Tromenie route follows a much longer and more arduous 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 such a sacred walk was made yearly during the B.C. years, but I wouldn’t bet against it. Some of the markers date to pre-Roman times. And the Gauls were people of strong traditions.
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.