Why is summer not in the twenty-four solar terms?

Because the twenty-four solar terms are determined according to the position of the sun on the yellow meridian, and the summer solstice is not a specific solar term. Twenty-four solar terms are the traditional solar terms system of China lunar calendar, which is used to divide the four seasons of a year and agricultural activities. It is determined according to the trajectory of the sun on the ecliptic, and each solar term has its own specific time point.