Is Tomb-Sweeping Day a traditional festival in China? Why the solar calendar?

Your observation is very detailed. The ancient calendar in China was very advanced. People sometimes call it "lunar calendar", but it is actually inaccurate. The traditional lunar calendar in China refers to the "Yin-Yang calendar", which is a customized calendar based on the northern moon and the 24 solar terms, and is used to guide agricultural production. Because people's physiological cycle is influenced by the moon, people's diet and activities are arranged according to the lunar calendar and the northern moon, and crop growth is influenced by the climate of the sun, so the twenty-four solar terms are actually one.

Because the 24 solar terms are formulated according to the operation of the sun, and they are the same solar calendar as the Gregorian calendar that we generally use today, the date of the solar terms is always only one or two days before and after the Gregorian calendar.