Gion Matsuri (Kyoto)
Gion Matsuri is a traditional festival that has been held every summer since 898. It started because of a belief that it would rid the country of the plague which had reached epidemic levels in the country. The main point of interest of this extremely popular festival in the old capital is the parading on public roads of decorated floats called Yamaboko. At night these floats have lit lanterns that look spectacular.
For details, please see the link below.
Tenjin Matsuri (Osaka)
Tenjin Matsuri is a festival where fire and water come together - it simply looks amazing and beautiful. At the end of the evening, some 100 boats sail on the river and fireworks are set off for the final show. This year, Tenjin Matsuri is happening on July 24th and 25th.
The festival combines beautiful ornated floats and fireworks! One of the rare summer festival in Japan showcasing these two traditional features.
Fuji Rock Festival (Niigata)
Fuji Rock Festival is one of the top three music festivals in Japan. It features every year for three days some of the praised rock music artists in the world. The outdoor music festival is a big summer celebration and camping event where you can meet many music lovers. For more information, please refer to the link below.
Sumidagawa Fireworks (Tokyo)
The Sumidagawa Fireworks Festival in Tokyo is a must-go event if you are in Tokyo during late July! The 2018 edition will be held on the 28th. This is one of the oldest and most impressive fireworks shows in Japan. The venue along the Sumida River makes it even more stunning. This event is popular, so get there early for a good spot because they'll be lots of people! Many of which will be wearing beautiful summer kimonos called yukatas.
Mitama Matsuri (Tokyo)
The stunning feature of this festival is the display of some 30,000 lanterns. The atmosphere at night is truly magical. There are also other things to enjoy such as Aomori Nebuta lantern floats and "Mikoshi-furi", where people swing traditional floats up and down.
Hakata Gion Yamakasa (Fukuoka)
The Hakata Gion Yamasaka is a hugely popular float festival that attracts approximately 1 million spectators every year in Fukuoka City. The festival has a 750-year history and is one of Fukuoka's most prominent. During this festival, men race through the city while carrying on their shoulders one ton floats called kakiyama.
Prior to the racing event, elaborately decorated floats are displayed around the city as a kind of pre-procession ritual.
Kagurazaka Matsuri (Tokyo)
Kakurazaka is a traditional part of the Shinjuku ward. Every year, at the end of July, the place becomes the stage for one of the most popular Awaodori (traditional dance) festivals in the country, the Kagurazaka Matsuri. Aside from the Awaodori, you can enjoy a great number of yatai (food stalls) offering typical festival foods. Wear a yukata (summer kimono) to really get into the festival's atmosphere!
In case you're going to Japan earlier and would like to know about the spring activities, here is a guide you might find useful.