Tokyo’s most famous firefly festival is in Fussa, a city that lies about 1 hour west on the rapid train from Shinjuku Station. About 500 fireflies are released in Fussa Firefly Park which is about a 25-minute walk south of Fussa Station. Between 8pm and 10pm is the best time to see the fireflies, but the festival official starts at 1pm with performances on the stage and the usual collection of street stands. Needless to say, the event is popular and it’s best to arrive before sundown if you want to secure a good viewing spot. See here for suggestions on other places to enjoy the fireflies in and near Tokyo.
Event details are taken from official sources whenever possible. It is always prudent to check the official event website before attending to make sure that there have not been any changes. If you know of an error in the calendar, then please let me know through the contact form.