Originally Posted by
chamade
Hi. If you've got flexibility don't plan on Japan during summer. It's gruesome especially from mid June through the first week of August, when it is constantly rainy and humid and yucky, and from August through mid September it's just hot and humid and exhausting.
Having said that, even in summer onsens are inviting as you can feel slightly refreshed afterward, for awhile anyway-- until the humdity outside again makes you feel sticky and damp.
If you are from a place with a dry summer, Japan will be a genuine challenge in summer, especially when you want to be outdoors. 
Two words: Japan Alps.
Three more words: Hokkaido.