20 17 August 4th of the lunar calendar corresponds to September 23rd, which is the autumnal equinox, the16th solar term among the 24 solar terms, and the time point is between September 23rd and September 24th of the Gregorian calendar every year, so the climate in the south only begins to fall from this solar term. The sun reaches longitude 180 degrees, pointing to the equator, and the northern and southern hemispheres are divided equally day and night. For our northern hemisphere, after the autumnal equinox, the day becomes shorter and the night becomes longer.

The three stages of the autumnal equinox are: "when the thunder stops, when the insects sting, and when the water dries up". As one of the "dichotomies", the autumnal equinox has been valued by officials and people since ancient times, and it has also formed a rich folk custom of solar terms, including sacrifice, diet, tourism and entertainment.

Dong Zhongshu said in the Spring and Autumn Story: "The autumnal equinox is half cloudy and half sunny, so it is cold and summer day and night." The vernal equinox actually means half. In the past, autumn was divided into three parts: Qiu Meng, Mid-Autumn Festival and Qiu Ji, and the autumnal equinox was on the Mid-Autumn Festival, so the so-called equal share was also true.