Once the rainy season officially ends, Japanese people dress up in kimonos and costumes and celebrate the season with a bunch of local festivals. Last year, I took a bunch of pictures of the Koshigaya festival. It takes place over two days in front of the Minamikoshigaya/Shinkoshigaya Station complex.
This year's festival is still about a month away, but now that I know what to expect I'll enjoy it even more and probably be able to learn more about it! But even without any knowledge of what's going on, visitors to Japan should try to see a festival while they're here.