Japan’s smallest botanical garden is located about a 10-minute walk from Shibuya Station and each year hosts a Firefly Evening (Hotaru no Yube) where about 500 are released. 17:30 to 21:00 each day (last admission 2015). Entrance is free. 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.