There is festival called BIg 3 in Japan. The bg 3 is basically TOP 3 matsuri in Japan. These 3 has long history plus, it is massive. Many peope attend every yaer and wellknown among Japanese people.
Kanda matsuri (Tokyo) Tenjin matsuri (Osaka) Gion matsuri (Kyoto)

There is many kind of summer festival in Japan. And one of the most famous is fire works festival! And at sumemr festival, you can eat and can have a fun lot fo thing!

Actually in Japan, fire works festival and summer festival is different festival. For fire works festival focus more to fire works but summer festival focus more to historical stuff like mikoshi. So both has very good poit! Just go both! And there is top 3 big fire works festival! which is.... Zenkoku hanabi kyougi taikai (Akita) Cuchiura zenkou hanabi taikai (Ibaraki) Nagaoka matsuri ohanabi taikai (Nigata))

Summer festival and fire works are usually held during summer. Alought, there is many other event held in other season too. Like in spring there is Sakura fes and in winter, there is snow fes. So, you can attend to traditional event every season!