Why are the 24 solar terms also calendars?

The twenty-four solar terms are actually a calendar. Since we have the concept of solar terms, people can know exactly what to do in farm work. Although the Gregorian calendar has been implemented in China for more than 90 years, proverbs and folk songs such as "Sowing in Qingming, Transplanting in Grain Rain" are still widely circulated among farmers, showing the vitality of the 24 solar terms. Twenty-four solar terms are the general term for solar terms and neutral qi. Among them, beginning of spring, Jingzhe, Qingming, Changxia, Mangzhong, Xiaoshu, beginning of autumn, Bailu, Hanlu, beginning of winter, heavy snow and slight cold are solar terms, while rain, vernal equinox, Grain Rain, Xiaoman, summer solstice, great summer heat, early summer heat, autumnal equinox, early frost, light snow, winter solstice and great cold are neutral. Solar terms are placed at the beginning of the month and in the middle of the month. It is stipulated in the lunar calendar that there are twelve Chinese Qi as the symbol of twelve months, and each month must have a fixed Chinese Qi. For example, the rainy month is the first month, the vernal equinox month is February, and the winter solstice month is November.