Eric: If it's to be in the Philly area, it probably shouldn't be the last weekend in August, since that is the weekend of the Philadelphia Folk Festival. The local folkies will be flocking to that as well as putting up their friends from out of town, and the non-local folkies will be either camping out at the Fest or clogging up the local motels, so it might be hard to find a place to stay in the area that weekend.

Actually, I'd vote for September in the Philly area: the weather is less likely to be terribly chilly, and the local folk scene is "between" the summer festival season and the regular autumn-spring concert season. June's nice, too.