The dog days, which appear between the hot summer and the hot summer, are the days with the highest temperature, humidity and sultry weather in a year. The "crouching" in "dog days" refers to "crouching evil", which is the so-called "six evils" in summer evil.

The traditional almanac stipulates that "counting from the third Geng day in summer", that is to say, "counting down from the summer day until the third Geng day" is about to begin to dormant. The "Geng Day" here refers to the days prefixed with "Geng" in the ancient "Ganzhi Jiri Law". "crouching" means being forced to hide underground; This also means that it is too hot to move.

The length of dog days mainly lies in the difference between middle and next days. Because the third Geng day (the first autumn) after the summer solstice solar term occurs at different times every year, the number of days in the middle autumn is also different. The incubation time is long or short, which may be 10 day or 20 days.

So there are two situations: some years are 30 days in dog days, and some years are 40 days.