Which solar term does the dog days refer to? How many days does a * * * have?

The dog days are the first, middle and last three days, which appear between summer and summer, and are the days with the highest temperature, humidity and sultry weather in a year. It appears from mid-July to mid-August in the solar calendar every year. In most years, the dog days are forty days.

According to the climate law of the lunar calendar in China, predecessors have long stipulated that "the third Geng day after long summer begins with the first day, the fourth Geng day begins with the second day, and the first Geng day after beginning of autumn ends with the third day, with the first ten days, the middle ten days, the twentieth day, and the third forty days. Every year in beginning of autumn and the next two days, if there is Geng Day, it is ten days in the middle, otherwise it is twenty days. So in most years, there will be 20 days in the middle, and correspondingly, most years will be 40 days.