Why is there Tomb-Sweeping Day?

The date in Tomb-Sweeping Day is relatively fixed in the Gregorian calendar. Most years are April 5, but this year is April 4.

Many people have an illusion: when they see that all the 24 solar terms are marked on the summer calendar of the calendar, they think that this solar term is "the lunar calendar". In fact, solar terms are an out-and-out "solar calendar" because they are divided according to the position of the sun on the ecliptic (actually the orbit of the earth around the sun). Depending on the sun starting from the vernal equinox (longitude zero, at this moment the sun shines vertically on the equator), every advance of 15 degrees is a solar term; After a week's operation, I returned to the vernal equinox, which is a tropical year with 360 degrees, so it is divided into 24 solar terms. When the sun reaches longitude 15 degrees, it is regarded as the moment of "Tomb-Sweeping Day", and this day is Tomb-Sweeping Day. From last year's Tomb-Sweeping Day to this year's Tomb-Sweeping Day, the length of the period is also equivalent to a tropical year, which is about 365 days, 5 hours and 49 minutes (a few minutes difference between years), so there is no question of Tomb-Sweeping Day's "advance".

So why is Tomb-Sweeping Day one day ahead of the Gregorian calendar this year? Last year, it was April 5 12: 52 in Tomb-Sweeping Day, and this year it was 365 days and 5 hours 5 1 in Tomb-Sweeping Day, so it fell on April 4 18: 43, and Tomb-Sweeping Day was on April 4. In the next three years, it will be April 5th in Tomb-Sweeping Day. The date of Tomb-Sweeping Day in the solar calendar has such a rule: whenever the solar calendar is a flat year (February 28), it falls on April 5; Whenever there is a leap year (February 29th) in the Gregorian calendar, it must be April 4th in Tomb-Sweeping Day. Tomb-Sweeping Day in 2008 was on April 4th.