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

The traditional almanac stipulates that "it will count at the third Geng day of the summer solstice", which means that "counting backwards from the summer solstice day and counting to the third Geng day" will begin to crouch. The "Geng Day" here refers to the day with the prefix "Geng" in the ancient "Ganzhi Jiri Law". "Fu" means that Yin qi is forced to hide underground by Yang qi; It also means that the weather is too hot to move.

The length of the dog days mainly lies in the difference between the middle and the lower days. Because the third Geng day (first fall) after the summer solstice solar terms occurs at different times every year, the number of days leading to the middle fall is also different. The mid-lying time is long or short, which may be 10 day or 20 days.

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