Where did the 24 solar terms first appear?

It was formed in the Spring and Autumn Period and the Warring States Period.

Twenty-four solar terms, a supplementary calendar to guide farming in ancient China, were formed in the Spring and Autumn Period and the Warring States Period. Because the lunar calendar in China is a kind of "yin and yang calendar", which is based on the movement of the sun and the moon, it can't fully reflect the solar cycle. But China is an agricultural society, and agriculture needs a strict understanding of the movement of the sun, so farming is entirely based on the sun. Therefore, the "twenty-four solar terms" that reflect the solar cycle alone are added to the calendar as the standard for determining leap months. Twenty-four solar terms can reflect the changes of seasons, guide agricultural activities and affect the food, clothing, housing and transportation of thousands of families. 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).