Why is Tomb-Sweeping Day either the 4th or 5th every year?

Because Qingming is based on the relative position of the sun and the earth to determine the date, when the sun reaches longitude 15 degrees, it is Tomb-Sweeping Day. And the earth's rotation around the sun is not an integer of 365 days. The rotation of the earth around the sun is 365 days, 5 hours, 48 minutes and 46 seconds, so the next day will be delayed every four years or so. For this reason, it is usually April 5th in Tomb-Sweeping Day, but sometimes it is April 4th and sometimes it is April 6th.

Qingming is the fifth solar term in the twenty-four solar terms of the lunar calendar, which indicates the official start of the Ji Chun season. When the sun reaches longitude 15 degrees, it will be Tomb-Sweeping Day gas. Tomb-Sweeping Day is one of the traditional festivals in China, and it is also one of the most important festivals to worship ancestors and sweep graves.