More about it : http://en.wikipedia.org/wiki/Fest_Noz
And there will be one in Paris on Saturday so if you want to join, just come, it is not only for Breton people.
It is 3 Euros at the door, to dance and enjoy music during 4 hours. And don't worry if you don't know how to dance, i don't neither and people usually are friendly enough to help :)
To find us inside or if the door is closed, you can call me : 07 81 72 98 67 and i will find you ;)
A benn disadorn !