One of the most beloved creatures in the town of Hidaka is the enchanting firefly. Fireflies typically begin gathering in late May to mid-June, but they are only around for a short timeframe. During this time you can attend Firefly Festivals in several places along the Hidaka River.
For the most up-to-date information on the next firefly season timeframe and viewing locations, please refer Japanese page:http://hidakagawa-kanko.jp/miru/hotaru.html
The information is typically updated in mid-May.