What is the origin of dog days?

The "dog days" were determined according to the "Ganzhi Jiri Law" in ancient China. The third Geng Day after long summer (referring to the day with the word "Geng") is the beginning, the fourth Geng Day is the middle, and the first Geng Day after beginning of autumn is the end. Together, it's dog days.

As the saying goes, "the heat is at three volts." Three volts refers to three consecutive periods of initial, intermediate and final voltage. From summer to the future, the third Geng day is the first day of initial dive, the fourth Geng day is the first day of intermediate dive, and the first Geng day after beginning of autumn is the first day of final dive, each dive is 1 day, but in some years, the dive lasts for 2 days, such as 25 years. Generally speaking, in the "dog days", the average temperature is the highest in the "middle range".