Michael, Photostock is in Emmet County, Michigan, primarily in Cross Village and Harbor Springs. As Andy correctly mentioned, it's around the solstice in June. It's an event organized by Bill Schwab, and has been a yearly event since about 2007.
I'm not sure that the event will move geographically. The area has a lot to offer photographically, and the environment is really relaxed, laid back, and conducive to creative bursts.