How did the ancients determine which day was Tomb-Sweeping Day?

In ancient times, the time in Tomb-Sweeping Day was different in different regions: Jin Wengong designated the second day of the Cold Food Festival as Tomb-Sweeping Day; In most parts of Shanxi, the day before Tomb-Sweeping Day was the Cold Food Festival. Tomb-Sweeping Day in Yushe County and other places celebrated the Cold Food Festival two days ago; Yuanqu County also stressed that the day before in Tomb-Sweeping Day was the Cold Food Festival.

The name of Tomb-Sweeping Day is related to the weather and climate characteristics at this time. The Western Han Dynasty's "Huainanzi Astronomical Training" said: "On the fifteenth day after the vernal equinox, the bucket refers to B, and the Qingming wind is coming." "Qingming Wind" is a refreshing and clear wind. "When I was in 100 questions" said, "Everything grows at this time, clean and bright. So it is called Qingming. "

Although Qingming, as a festival, was only formed in the Tang Dynasty, Tomb-Sweeping Day Qi, as a symbol of time sequence, has long been known by the ancients and clearly recorded in the Han Dynasty. Tomb-Sweeping Day Qi provides important conditions for the formation of Tomb-Sweeping Day customs in terms of time and weather, and this solar term is considered as one of the origins of Tomb-Sweeping Day.