My wife and I do our big trips in August, usually the last 3 weeks or so before Labour Day.  It's a glorious time of year - plenty of sun, cool but not cold nights, few bugs and, if you know where to look, the fishing can be pretty good excellent too.  I also tend to do a shorter trip around the end of September or early October.  "Indian Summer" just after the first frost when summer time weather comes back for a brief and final visit is also a wonderful time of year especially if you hit it when the colors change.