One of my favourite memories of Japan, to this day, is seeing fireflies last year in a rural part of Kagawa prefecture, outside of Marugame. I remember picking them up and watching them glow- it was my first time ever seeing fireflies, and it's such a beautiful memory.
Hotaru ホタル, as they call fireflies in Japanese, only come for a very short time in June. If you live in the countryside, I highly recommend going to explore around the rice fields and clear water late at night, as it's such a beautiful experience. In the cities, you may need to get a bit more creative, but some parks and river banks surely host fireflies.
In Japan, I love how seasonal things, such as cherry blossoms, leaves changing colours, or fireflies are celebrated events.