Why did Tomb-Sweeping Day change the date?

There is a folk saying in China that "Tomb-Sweeping Day was in April 5th", that is, Tomb-Sweeping Day was on April 5th. Astronomers say that the date in Tomb-Sweeping Day actually changes from April 4th to April 6th every year, and it is not fixed on April 5th.

Because the length of a year is determined by the movement of the sun, and the 24 solar terms are also determined by the position of the sun, basically the date of each solar term will not change much every year. However, the actual length of a tropical year is 365.2422 days, and the Gregorian calendar year is 365 days, so there will be 0.2422 days (equivalent to 5.8 hours) every year, and the specific time of solar terms will be "delayed" by 0.2422 days every year; As a result, it accumulated to 0.9688 days in four years, almost one day. In order to correct it, there is a "leap year" system in the Gregorian calendar, with one more day on February 29th every four years. If the date of a solar term is delayed from midnight to the next day or advanced to the previous day because of a leap year, the date of the solar term will change, but the date difference will only be within 3 days.

According to astronomical experts, in recent decades, the date of Qingming has been changing from April 4 to 6. Statistics show that the Qingming date of 190 1 to 1943 is April 5 or 6; April 5 1944 to1975; 1976 by mid-2000, there was a leap year on February 29th, April 4th in Tomb-Sweeping Day, and 5th in other years; In the 20 years since 2008, every leap year and the Qingming Festival of the following year are April 4. For example, in 2008 and 2009, 20 12 and 20 13, 2020 and 202 1, the Qingming date in 2024 and 2025 is April 4.