Japan in the summer

My suggestion would be to go as early in the month of June as you can manage - the weather then in both Tokyo and Kyoto will still be relatively temperate and pleasant for day tripping.
Things start to get pretty toasty by the middle of July, and by August the big cities are starting to become uninhabitable.

Don't do Japan in the Summer. We did it in the Fall and it was nice.

June can be rainy and then it starts to get humid until the humidity breaks in late August/September. But go when you can go. Mid-June would be fine.

We could only do June/July as we were traveling with our high school and college aged students - we did mid June into early July. It was hot but fine. Mid 80’s was the highest we saw. Go and enjoy it!

Japan is worth visiting even if you are limited in time of year. But the earlier you can go within your time frame, the better it would be.

Fortunately, it seems you are never more than a block or two to the nearest convenience store or vending machine if you need a cold drink.

Because I was a teacher at the time, we had to visit Japan in the summer. We traveled there for 3 weeks, late July/ early August. In addition to high temps, the humidity was very high, over 90% every day. One day it hit 99%. 1987, there was very little ac. The yen was very strong vs the usd, so no luxury accommodation. One ryokan had a coin-op ac, which we used to get the room cool enough to get to sleep. Mid-June might be better.
Despite the discomfort, we loved that trip. I still think of it as one of the best we’ve ever taken.

My two work trips to Japan were in August and the heat and humidity were oppressive, so I agree with those who suggest the earlier the better. I took a few extra days to see some sights and I still had a great time, despite the heat.