There are four seasons in Japan, however, the changes between the four seasons are not clear and very small. June is the rainy season with a lot of rain and it’s over in July, then it changes to a midsummer season.
Japanese summers are very hot and usually are high in humidity and damp. Some people die of heat stroke every year if they don’t drink water frequently to avoid dehydration. I recommend preparing the battle with heat by being hydrated for your trip to Japan in summer. Since minerals are lost with water, it’s good to take salt with your water too.