The Firefly Festival takes place at Hotaru Park in Fussa, Tokyo, on June 11 from 1 p.m. to 8 p.m. Admission is free. The park is a five-minute walk from Ushihama Station on the Ome Line and Kumagawa Station on the JR Itsukaichi Line.

Large crowds usually visit the Hotaru Park to watch the firefly show of lights on the Fussa River’s banks…said to be best from around 7 p.m. There will also be a whole host of traditional food stalls and other festivities taking place along the town’s streets throughout the day.

Situated next to the city’s meandering Tsuri River, the Hotaru Park (“hotaru” is Japanese for “firefly”) is notable for their abundant amount of fireflies, and there is a special hotaru no yakata (firefly house) to watch the seasonal display from. It is said that the fireflies are bred by a small group of locals and secretly released for the festival.

The Fireflies can be seen for a few more days than the one-day official event. Note:  The event is subject to dry weather.

Source: Fussa river is abuzz with light by Mike Hamilton (Japan Times)
Find out about the Breeding Center at the Hotaru Park here.