Can confirm that Breton Festou Noz are frequently a good bet. Lots of them are associated with "Pardons" which are celebrations of a church saints day. So you might get a church service followed by a procession with Breton dancers in traditional costume. The dancers are quite often bussed in from elsewhere.

We attended a small pardon near Pontivy which involved a procession to a spring. The little shrine over the spring featured a statue resembling Noggin the Nog - a huge bonfire was ignited and blessed by the priest (C of E it ain't). As it was the same week as the Tour de France, at the subsequent beanfeast there was a tableau featuring scarecrows done up in Lycra holding elderly bikes (and a dog show).

We also attended a huge pardon and fest noz near Auray where there were about ten lots of dancers, bombardes, binious, Gascons with very silly shoes, the lot! The fest noz in the evening featured more bombardes, binious and Le Sonerien Du, plus lots of assorted food and rotgut! Bliss.