here is PFS address, They can direct you to what is happening in the Portland area. There is a music festival in Richland Wa Labor Day week end. That is a 280 mile east of Portland. There is Silver Falls fest out of Corvalis OR. That is a music campout week end.
http://community.gorge.net/mcfs/
http://www.3rfs.org/
Try some of these sites where they may offer some information for you. There is another festival that comes to mind in September, that is in Sisters OR which is in the middle of the state near Bend OR.
Good luck....
Mudjack