When was the solar term "Long Summer" determined?

Summer is a traditional folk festival of the Han nationality, called "Long Summer Festival", which is popular all over the country and is held in April of the lunar calendar (May 6th in the solar calendar). The long summer, like the beginning of spring, the beginning of autumn and the beginning of winter, marks the beginning of the four seasons. When the sun in Xia Zhishi reaches 45 degrees, it is the moment when summer begins. Like the eastern suburbs in early spring, there are ceremonies to welcome summer in the long summer. In ancient times, Emperor Tiandi led hundreds of officials to welcome the summer in the southern suburbs, offering sacrifices to Emperor Yan and Zhu Rong. Long summer is also an important solar term. People measure the abundance of a year with the eyes of dark clouds in midsummer, and think that it rains best when it rains in long summer. There is a folk saying that "it doesn't rain in the long summer, and wheat comes in the drought" and "it doesn't rain in the long summer, and the plow hangs high".