Why is the number nine 8 1 day and the summer heat is 40 days?

Because the number nine is based on nine days, and the summer heat is calculated by Geng day (every 10 day is a Geng day).

Jiujiu, also known as Winter Jiujiu, is a solar term of the Han people, which was very popular in the Northern and Southern Dynasties. In the traditional culture of Han nationality, nine is the extreme number, which is the largest, largest and longest concept. Nine nines is eighty-one, which is the "maximum" number. Ancient Han people thought that September 8 1 day must be the spring after the winter solstice.

Summer lodging, also known as "three lodging", is the manifestation of the hottest period in a year in China's old calendar, which can be divided into initial lodging, intermediate lodging and final lodging. According to the climate law of China lunar calendar, predecessors have long stipulated that the third Geng day from long summer to the future is Fushou, the fourth Geng day is Fuzhong, and the first Geng day after beginning of autumn is Fuwei. There are ten days in the first and last days, ten or twenty days in the middle, and thirty or forty days in the summer.

Every year, if Geng Day falls on beginning of autumn Day and two days later, it is ten days in midsummer, otherwise it is twenty days. Therefore, the summer in most years is twenty days, and correspondingly, the summer in most years is forty days.