To enter the ambush means to enter the dog days. As the folk saying goes, "Three Geng's head falls in the summer solstice", which is the basis for establishing the first fall. The almanac stipulates that "the number of volts will be counted on the third day of the summer solstice", which means that "counting backwards from the summer solstice" and counting to the third "Geng day" will begin to fall. The "Geng Day" here refers to the day with the prefix "Geng" in the ancient "Ganzhi Jiri Law".

The date of Geng Day is changeable and not fixed in the Gregorian calendar, so the date of lodging is different every year, but the date of lodging is always between July 1 1 day and July 20 in the Gregorian calendar. The fourth Geng day to the fifth Geng day is middling; The first Geng Day to the second Geng Day after beginning of autumn is the last one. Every Geng day is 10 days apart, and the number of midsummer days is not fixed. There are four Geng days between the summer solstice and beginning of autumn, the midsummer days are 10 days, and there are five Geng days, and the midsummer days are 20 days.

Introduction to the calculation method of the lodging time;

In the lunar calendar used in China, there is a kind of "branch calendar method", which uses the words "heavenly stem" (A, B, C, D, E, Ji, G, Xin, Ren, Gui) 10 and "earthly branch" (Zi, Ugly, Yin, Mao, Chen, Si, Mao, Mao, Mao, Mao, Mao, Mao, Mao, Mao, Mao, Mao, Mao, Mao, Mao

According to the provisions of the "Cadre and Branch Day Law", the date of the dog days is from the summer solstice to the third Geng day, which is the first lodging, the fourth Geng day to the fifth Geng day is the middle lodging, and the first Geng day to the second Geng day after beginning of autumn is the last lodging. Every Geng day is 10 days apart, and the number of midsummer days is not fixed. There are four Geng days between the summer solstice and beginning of autumn, the midsummer days are 10 days, and there are five Geng days, and the midsummer days are 20 days.