In ancient times, it was called summer solstice and winter solstice.

In ancient times, the summer solstice and the winter solstice were called "the second solstice". "Zhi" means extreme and most. Therefore, the summer solstice and the winter solstice are collectively called "the second solstice", which means the acme of summer and winter. Summer solstice and winter solstice are generally on June 2 1 day and February 22 12 of the Gregorian calendar every year. On the solstice of summer, the sun shines directly at 23.5 degrees north latitude and 90 degrees east longitude, which is the longest place in the northern hemisphere during the day. On the solstice of winter, the sun shines directly at 23.5 south latitude and 270 east longitude, which is the shortest day in the northern hemisphere.

Twenty-four solar terms are precious scientific heritage created by China ancestors through thousands of years' practice. They are tools to reflect the changes of weather, climate and phenology and master the farming season. It originated in the Yellow River Basin. As early as the Spring and Autumn Period and the Warring States Period, China was able to measure the length of the midday sun shadow with a soil gauge (a pole was erected on a plane), and four solar terms were determined according to the length, namely, winter solstice, summer solstice, vernal equinox and autumn equinox. The Book of History in the Spring and Autumn Period recorded the solar terms. After continuous improvement and perfection, by the Qin and Han Dynasties, the 24 solar terms had been completely established. The twenty-four solar terms are completely recorded in Huainanzi written by Liu An in the Western Han Dynasty. In BC 104, the taichu calendar written by Deng Ping officially set 24 solar terms in the calendar, and defined the astronomical position of the 24 solar terms.

The twenty-four solar terms reflect the movement of the sun, so the date of the solar terms is basically fixed in the current Gregorian calendar. The first half of the year is on the 6th and 2nd1,and the second half is on the 8th and 23rd, with a difference of 1 ~ 2 days.