Why is the Gregorian calendar in Tomb-Sweeping Day? That's true.

Traditional festivals such as Spring Festival, Dragon Boat Festival and Mid-Autumn Festival all have fixed lunar calendar days, but not in Tomb-Sweeping Day. Why does Tomb-Sweeping Day use Gregorian calendar every year?

It turns out that, unlike most traditional festivals in China, Qingming Festival is one of the 24 solar terms, and the time depends on the trajectory of the sun. It is a sign of phenological changes and seasonal order in China, belonging to the solar calendar. The twenty-four solar terms are divided according to the position of the sun on the ecliptic (that is, the orbit of the earth around the sun). Therefore, the dates of solar terms in the solar calendar are basically fixed. Every year in Tomb-Sweeping Day, it is around April 5, with a difference of only one or two days, and the date of the lunar calendar changes greatly.

According to reports, since the Qing Dynasty, the vernal equinox has been set at 0 degrees, and every time the sun runs 15 degrees on the ecliptic, it is set as a solar term, so 24 solar terms are 24 specific moments, not 24 specific days. 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, the date of each solar term does not change much in each year.

However, the actual length of a tropical year is 365.2422 days, and the length of a Gregorian calendar year is 365 days, so there will be 0.2422 days more each year (equivalent to 5.8 hours), and the specific time of solar terms will be postponed by 0.2422 days each 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 solar calendar, with an extra day on February 29 every four years. Due to the delay, the date of solar terms will be changed to the next day after midnight, or one day earlier due to leap year, but the date difference will only be within 3 days.