Why is the date in Tomb-Sweeping Day not the lunar calendar?

Tomb-Sweeping Day is one of the 24 solar terms.

A year is divided into 24 solar terms according to the movement of the sun and several months according to the movement of the moon. Twelve miscarriages and big months add up to only 354 or 355 days. Compared with the two, the difference is about eleven days. In order to coordinate the two, the ancients adopted the method of "fixed leap", and one year in several years was thirteen months (leap month). Our 24 solar terms are not based on the lunar or solar calendar. Strictly speaking, they are based on the longitude of the sun. The path of the sun under the starry sky background is called the ecliptic. When the sun moves to the longitude of 0 degrees, it is the vernal equinox, which is a solar term of about fifteen days. By analogy, "Qingming" is when the sun moves to longitude 15 degrees. Because of the leap year, the date of solar terms will change, but the date difference will only be within three days, and the date in Tomb-Sweeping Day will be between April 4th and 6th in the Gregorian calendar. For example, April 5th is in Tomb-Sweeping Day this year, and April 4th is in Tomb-Sweeping Day in 2009. Other traditional festivals, such as Dragon Boat Festival, Mid-Autumn Festival and Spring Festival, are based on the lunar calendar, so the Gregorian calendar dates vary greatly every year.