Tomb-Sweeping Day is not fixed every year. I wonder how it is determined.

The twenty-four solar terms, including Qingming, are determined according to the position of the earth on the orbit of revolution, which can be accurate to a few minutes and seconds, basically corresponding to the date of the solar calendar (many people think that the twenty-four solar terms follow the lunar calendar, which is a misunderstanding. You just need to compare the calendars of different two years casually: solar terms follow the solar calendar. However, the solar calendar is about 6 hours less than the earth's revolution (so one day is added at the end of February every four years to make up for this time). Because of this error, the dates of solar terms (including Qingming) will fluctuate for a day or two in the solar calendar.