The ambush begins on the third Geng day after the summer solstice. The first period lasts for ten days, that is, the middle period starts from the fourth Geng day after the summer solstice. Mofu begins on the first Geng day after the Beginning of Autumn, and the number of days is also ten. Between the first and last volts is the middle volts. Due to the different occurrence times of Geng Day, the time of the mid-voltage period is also not fixed. In some years, it lasts for ten days, and in other years, it lasts for twenty days.
According to the calculation method of the Heavenly Stems and Earthly Branches, every day with "Gen" is a Gen day. There are ten heavenly stems, in order: A, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren and Gui; there are twelve earthly branches, in order: Zi, Chou, Yin, Mao, Chen, Si and Wu. , Wei, Shen, You, Xu, Hai. Because there are ten heavenly stems, every ten days is a Geng day.
"Three Volts" is the collective name for the first, middle and last periods, which lasts from mid-July to mid-August. Because the Zhongfu is not fixed, the Sanfu lasts for thirty days in some years and forty days in other years. Summer is the hottest time of the year, while Zhongfu is the hottest time of the year.