Summer in Japan is often hot and humid, but it's terribly fun because of all the fireworks festivals and other festivals going on. Summer kicks off mid July and goes until the end of August, coinciding with the school calendar.
It's a great time to wear yukata ゆかた (Japanese summer kimono) and eat festival foods and fish for gold fish 金魚すくい.
The big fireworks festivals are on the weekends but some smaller ones are even held on weekdays.
If you go to a big fireworks festival bring cash and go early with a sheet to claim a place to sit. People will start grabbing spaces the night before or even earlier.