Kugayama’s annual firefly festival has been going for over twenty years now and is typically held on the first weekend of June. The festival official starts at 1pm, but of course it is most lively after sunset as people crowd along the banks of the Kanda River and the Tamagawa Josui River in the hope of seeing the fireflies dance. The festival itself is a small-scale, community affair with the shops and stores that line the shotengai (commercial street) setting up tables outside to sell beer, yakitori, and candy floss.
Those coming in the hope of seeing a mass of glowing fireflies may be disappointed. The best spot to see them is to the south along the Tamagawa Josui River, but you need to peer through branches and undergrowth to see them intermittently glowing in the distance. From the path of the Kanda River the few fireflies that are to be found quite far away and even with the street lights off the river itself is fairly well illuminated, meaning that their glow doesn’t stand out so brightly. With that said, Kugayama Station is only 12 minutes on the rapid train (Keio Inokashira Line) from Shibuya which makes the firefly event one of the more convenient to reach in Tokyo. The festival runs from 1pm to 9pm on Saturday and Sunday. See here for suggestions on other places to enjoy the fireflies in and near Tokyo.