Originally Posted by pinniped
As for Western Europe, I think the airlines consider May 15 - Oct 15 as the high season. The big hotel chains are probably similar... However, I have spent time all over Europe in September and loved it: I didn't feel that September was a bad month to be there at all. (I get the sense that max-peak tourist season is more like July-August.)
I have been to Europe in May and early June as well as Sept., and feel it is also a good time to travel -- not very crowded, pretty good weather, etc. It may be considered high season with respect to prices, though -- I can't be sure about that. I agree that July-August is definitely the highest of high season in Europe and should be avoided if at all possible.