Until what period, the second solar term of 14 was completely formed.

According to textual research, the names of the twenty-four solar terms first appeared in the Western Han Dynasty document Huainanzi Astronomical News. According to historical records, by the Western Zhou Dynasty, the ancients had identified four solar terms: winter, summer, vernal equinox and autumnal equinox, and by the Warring States period, a complete 24 solar terms were basically formed. During the period of Emperor Wu of the Han Dynasty, the twenty-four solar terms and the Song Dynasty were incorporated into the calendar as a supplement to guide farming.

The current 24 solar terms originated from the division of the "fixed gas law" concluded more than 300 years ago. The method of "fixing qi" is a method to determine solar terms according to the position of the sun on the ecliptic. The solar terms are divided from beginning of spring to the end of Great Cold. Twenty-four solar terms are the time standards promulgated by successive governments, and also the guide for people to engage in agricultural production.