The viewing of fireflies at Kinchakuda is one of the earliest firefly (hotaru in Japanese) events in Saitama that I know of. You can see them as early as the end of May, although its usually early June. With the weather the way it has been, this is one of the years that the event will start end of May.
The firefly event for Kinchakuda dates for 2018 have been set as May 27th to June 4th inclusive. During this period the park will be open from 7.30 pm to 9 pm to facilitate firefly viewing. The firefly area is clearly marked and sectioned by bright lights around the periphery.
Location of the fireflies at kinchakuda, from the official site
The park costs 300 yen in during the red spider lily season for which the park is most famous, but it is free in for the firefly event. Parking normally costs 500 yen, but according to the city website if you tell the staff at the information tent you are there for the firefly viewing the cost is wavered.
Full information on the afore mentioned red spider lilies:
Kinchakuda Red Spider Lily Festival | HIDAKA
And nearby in Hanno on the weekend of June 2nd and June 3rd there is the Hanno Green Carnival in the very cool Tove Jansson Akebono Children’s Forest.
Hanno Green Carnival at Akebono Children’s Forest | HANNO
Access Fireflies at Kinchakuda
15 minute walk from Seibu Ikebukuro Line’s Koma Station.
18 minute drive from Tsurugashima Interchange of Kanetsu Expressway.
MAP, URL, CONTACTS in the grey details box at the very bottom of the post.
Another event in the area on June 2nd
Seibu Train Festa 2018 | HIDAKA
In the area:
Easy Hike to Gojyou Waterfall | HIDAKA
Even more attractions in Hidaka City:
Hidaka Sougou Park | HIDAKA
A fun day out in Saiboku; Hot springs and Ham! ｜HIDAKA