Japan has 4 festivals in this guide, spread across 3 cities. They run in May, July, August and September, and festival-day temperatures range from 22°C (72°F) to 32°C (90°F), with nights down to 12°C (54°F). That range is the reason there is no single "Japan festival outfit". The answer changes by month and by site.
2 of them run in real heat (28°C or more), where loose natural fabrics beat coverage and a brimmed hat does more than any garment. 1 carries a high rain risk, which makes waterproof footwear a requirement rather than a precaution.
The ground matters as much as the sky. You will find mountain, arena grounds and city streets, which puts broken-in trainers or a low boot at the top of the list for most of these events. 2 of the 4 have camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans gorp, camping, kawaii and techno. Summer Sonic is the largest of them at around 150,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.