How did Tomb-Sweeping Day calculate it?

Tomb-Sweeping Day is not calculated according to the lunar calendar, but according to the ecliptic.

"Huainanzi Astronomical Training" says: "Fifteen days after the vernal equinox, the bucket refers to B, and the Qingming wind is coming." According to the centenarian question, "everything grows clean and bright at this time." So it is called Qingming. " As soon as Qingming arrives, the temperature rises and the rainfall increases, which is a good time for spring ploughing and planting. Therefore, there is an agricultural proverb that "before and after Qingming, point melons and plant beans" and "planting trees is not as good as Qingming". It can be seen that this solar term is closely related to agricultural production.