How to count "in ambush"?

Starting from the third Geng day after the summer solstice, the ambush begins.

In ancient China, heavenly stems and earthly branches were used to record time. The numbers of heavenly stems mainly include A, B, C, Ding, Wu, etc., while the numbers of earthly branches are Zi, Chou, Yin, Mao, etc. When the heavenly stems are matched with the earthly branches, we get Jiazi, Yichou, Bingyin, Dingmao, etc., which cross and cooperate sixty times, so it is called Yijiazi. "Three Geng" means that when you encounter three "G" characters, you will fall into ambush on the third Geng day, which is the first ambush.

The date of Geng Day changes and is not fixed, so the date of entering the ambush is different every year, but the date of entering the ambush is always between July 11 and July 20. The period from the fourth Geng day to the fifth Geng day is the middle period, and the first Geng day to the second Geng day after the beginning of autumn is the last period.

Each Geng Day is 10 days apart, and the number of Geng Days is not fixed. Between the summer solstice and the beginning of autumn, there are 4 Geng Days, and there are 10 Geng Days. There are 5 Geng Days, and there are 20 Geng Days. sky.