Why are the winter solstice, summer solstice, vernal equinox and autumn equinox fixed every year? What's the date of the solar calendar?

Solar terms represent the position of the earth in the orbit of revolution. Starting from the vernal equinox, every 15 degree is a solar term, and there are 24 solar terms in the tropic of cancer. According to the order of four seasons: spring: beginning of spring, rain, sting, vernal equinox, Qingming, Grain Rain; Summer: Long summer, Xiaoman, Mangzhong, Summer Solstice, Xiaoshu, Dashu; Autumn: beginning of autumn, early summer, Bailu, autumnal equinox, cold dew and early frost; Winter: beginning of winter, light snow, heavy snow, winter solstice, slight cold and severe cold. Therefore, solar terms represent the solar calendar, which basically conforms to the Gregorian calendar date, and because the solar day 1 tropic year =365.2422 is not an integer, it cannot be fixed. February, March, June, September winter solstice, vernal equinox, summer solstice and autumn equinox are generally 2 1 or 22. Generally speaking,