The whole sentence is: "On the day of summer, rotten grass turns into fireflies, and the next five days will be moist in summer, and the next five days will be fine with heavy rain." In the Book of Rites, "The moon is in summer, the sun is in willow ... The warm wind begins, the cricket lives on the wall, the eagle learns, and the rotten grass is firefly." This paragraph explains.
"On the hot summer day, the rotten grass is firefly, and the soil is wet in the next five days, and it rains heavily in the next five days." This sentence describes three phenology of the great summer heat: during the great summer heat, fireflies hatch, and the ancients thought fireflies were made of rotten grass; Five days later, the weather began to get sultry and the land was wet. In five days' time, there will often be heavy thunderstorms, and the heavy rain will reduce the heat and humidity, and the weather will begin to transition to beginning of autumn.