What day is Tomb-Sweeping Day? Why are the days different every year?

Because this is a solar term, not a festival.

It depends on the position of the sun, because the earth's rotation is biased, so it is different every year, but the difference is only one or two days.

It is April 4th in Tomb-Sweeping Day this year. Every year is usually on April 5, but it also falls on April 4 or 6. The 24 solar terms are a reflection of Heliocentrism, which is determined by the solar calendar and the position of the earth when it revolves around the sun. One revolution of the earth around the sun is 360 degrees of the yellow meridian, and when the earth reaches zero degrees of the yellow meridian, it is designated as the vernal equinox, and then there is a solar term every time it runs 15 degrees, and * * * has 24 solar terms. Therefore, the dates of solar terms in the solar calendar are fixed, with a difference of one or two days due to leap years. The lunar calendar invented by our ancestors is the product of geocentric theory. Because geocentric theory fails to grasp the original laws of things, it is quite troublesome to calculate, but there are solar terms reflecting the revolution of the earth around the sun in the lunar calendar, which are closely related to agricultural activities, which is also a great contribution.