From Jun 11, 2026 to Jun 15, 2026, the corresponding solar term in the Nijūshi Sekki (24 solar terms) is 芒種(Bousyu), and the corresponding microseason in the Shichijūni Kō (72 microseasons) is 腐草為蛍(Kusaretaru kusa hotaru to naru).

What Is “Kusaretaru kusa hotaru to naru” (腐草為蛍)?

"Kusaretaru Kusa Hotaru to Naru" (腐草為蛍, "Rotting Grass Becomes Fireflies") corresponds to the 26th phase of the Shichijūni Kō (72 microseasons). It is the second microseason of Bōshu (芒種) within the Nijūshi Sekki (24 solar terms), and it arrives each year around June 10–15. This microseason's evocative name literally means "rotting grass turns into fireflies." It marks the time when fireflies begin appearing one by one near riversides and rice paddies. Against the humid darkness of the rainy season nights, their soft lights drift gently through the air before fading away again. For centuries, this fleeting glow has been cherished in Japan as one of the most dreamlike symbols of summer. Before the advancement of science, particularly prior to the Edo period, people believed that fireflies were born from decaying grass or bamboo roots. Their mysterious light—glowing eerily in the darkness without giving off heat—must have seemed to people of the time like the very spirit of plants made visible. In reality, firefly larvae spend nearly a full year underwater before burrowing into the soil on rainy days to pupate. Then, during the rainy season, the adults emerge and take to the night sky, their tiny lives shining brilliantly for only a week or two. The word fusō (腐草, "rotting grass") was also once used as another name for fireflies themselves. The ancient people who observed fireflies rising from grassy thickets gave birth to this beautiful and mysterious seasonal phrase through their keen sense of the natural world. Around this time of year, the bell-shaped hotarubukuro flower blooms near the water's edge, heralding the season of fireflies. Its name is said to come from the old custom of children placing fireflies inside the blossoms while they played. Places where fireflies can still thrive—requiring both pure water and deep darkness—have become increasingly rare today. Perhaps that is precisely why their gentle light feels more precious and beautiful than ever.

List of Japan’s 72 Microseasons