Why does the traditional festival Tomb-Sweeping Day belong to the solar calendar?

The traditional calendar in China is a combination of yin and yang, and the days are usually calculated according to the lunar calendar, but agriculture should make a supplementary calendar according to the solar calendar, and the 24 solar terms are the supplementary calendar. Qingming is one of the 24 solar terms, so it is calculated according to the solar calendar. The twenty-four solar terms and the Gregorian calendar are both Gregorian calendars, but there are still differences in the way of calculating days. So Tomb-Sweeping Day is not fixed on April 5th in the solar calendar, but wandering back and forth between April 4th and 6th.