here is info for Dalarna (Dalakarlia)
http://www.sifferbostugby.se/placesofinterest.html
and the amphitheatre there..Dalhalla
which is in an old limestone quarry

http://www.dalhalla.se/

and here is a list of festivals
http://www.kadmusarts.com/countries/Sweden.html

(general festivals not only folk)