Hunting for fireflies in Kyoto, Japan is a great thing to do in June! Unfortunately, the exact timing within the season is not predictable, so multiple visits to known firefly sites may be needed.
Interestingly, it is a very well known event in Japan, and even outside of Japan, but I find it extremely difficult to find any pertinent information on the big web we call The Internet.
There probably are quite a few informative site in Japanese... But without knowing Japanese, it is quasi impossible to extract any useful information from those.
One common piece of information that can be found for Kyoto is The Philosopher's Path, and that is where the photos below are from.