Why is the Spring Festival not a fixed day?

First of all, the Spring Festival is different from the beginning of spring. Spring Festival is a lunar festival in China, and beginning of spring is a solar term. The solar terms are arranged according to the law of the sun's revolution, and the current Gregorian calendar is also compiled according to the sun's revolution, so the solar terms and Gregorian calendar dates are relatively fixed, which is why Qingming is relatively fixed between 4 and 6 days every year. Spring Festival is a lunar festival, which is the first day of the first month of every year. The lunar calendar is arranged according to the cycle of the moon circling the earth, which has nothing to do with the solar calendar, so sometimes it feels like a big span compared with the solar calendar.