How to calculate the dog days?

From summer to the third Geng day in the future, the beginning of the dog days is 10 days, the middle is 10 days or 20 days, and the end is 10 days.

The dog days appear in midsummer and summer heat, and it is the hottest, humid and sultry day of the year. The so-called "dog days" refers to the "dog days" of the lunar calendar, which is the hottest period of the year.

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.