The time of the ambush is not fixed every year, and the length of the ambush is also different. You need to consult the almanac to calculate it. You can simply use the four-character formula "Summer Solstice San Geng" to express the date of the ambush, that is, from after the summer solstice. Counting from the third "G" day, the first fall lasts for 10 days, the middle fall lasts for 10 or 20 days, and the last fall lasts for 10 days.

Starting from the summer solstice, according to the arrangement of the stems and branches, the third Geng day is the first fall, the fourth Geng day is the middle fall, and the first Geng day after the beginning of autumn is the last fall. When there are 4 Geng days between the Summer Solstice and the Beginning of Autumn, the Zhongfu period lasts for 10 days, and when there are 5 Geng days, the period lasts for 20 days. It seems that the sooner or later the Geng Day appears will affect the length of the Dog Day. Therefore, in some years, the Dog Day will last for 30 days, and in other years, the Dog Day will last for 40 days.

The three dog days are ten days, and one dog day is ten days. This is calculated according to the ten numbers of the heavenly stems. Therefore, the division of the dog days is as follows: the Geng day of the third celestial stem after the summer solstice is the beginning of the first dog day.

For example, the summer solstice in 2006 was on June 21 of the Western calendar year. The Heavenly Stem Day on this day was Xin Day. 28 days later, it was the third Geng Day. Therefore, July 2006 The 20th is the beginning of the first volt; ten days later, it is another Geng day, that is, July 30, and it enters the second volt; Ten days later, it is another Geng day, and it is the first day after the beginning of autumn. On the Geng day, August 9th, we entered the third volt.

If it is 2007, July 15th is the third Geng day, which is the beginning of the first volt (the first volt); ten days later, it is another Geng day, which is July 25. It enters the second period (Zhongfu); ten days later, it is another Geng day, but it has not yet passed the Beginning of Autumn. After another ten days (there are 20 days of Zhongfu this year), it is another Geng day. Moreover, it is the first Geng Day after the Beginning of Autumn, that is, August 14, which enters the third volt. This means that every year after entering the Dog Days, it is very hot, especially the ten days (sometimes twenty days) of the Dog Days, which are the hottest.