What is the saying that it rains from summer to Sunday?

It rains on the day of summer solstice: "if it rains on summer solstice, it should fall earlier."

Entering the summer solstice solar term, if it rains, it often indicates that autumn will come early, which means that this summer is cooler, not a hot and difficult weather.

Then why is it cool in summer? It's actually quite understandable. Through proverbs, we know that it rains from summer solstice to summer, so the weather is naturally cool and will not be exposed to the sun, hot and high temperature. So rainfall has a cooling effect.