The solar term is the lunar calendar.

The twenty-four solar terms are calculated according to the solar calendar. It is the general name of the twelve solar terms and twelve qi in a year, and it is the date when the sun passes through twenty-four specific positions on the ecliptic in a tropic year (the year calculated by the solar calendar). China used to use the lunar calendar (also known as lunar calendar, ancient calendar and old calendar) for a long time, with twelve moons as a year, and its length was about eleven days different from the tropic year. Set seven leap months in nineteen years, so that their periods are consistent with the tropic year. Therefore, there may be a difference of about half a month between the lunar month and the actual season, which is not conducive to guiding agricultural production. Therefore, astronomers in ancient China named seasons, climate and agricultural production to reflect the seasonal climate and guide agricultural time. Twenty-four solar terms originated in the Yellow River Basin. Among them, the four "Li" (beginning of spring, long summer, beginning of autumn and beginning of winter) is the beginning of the four seasons, and the equinoxes (vernal equinox and autumnal equinox) and two (summer and winter) are the first half of the four seasons. Twelve solar terms and twelve gases are alternately arranged. In order to make it easy to remember, people weave the order of the 24 solar terms in a year into a jingle: spring is two surprises, spring is bright and clear, summer is full and summer is hot, autumn is cold and frosty, winter is snowy and winter is cold.