The dog days refer to the period from the third Geng day after the summer solstice to the day before the second Geng day after beginning of autumn, which is the hottest period of the year in the Central Plains. The period is from mid-July to mid-August every year, with 3 days in some years and 4 days in others.

Three volts can be divided into initial, intermediate and final volts, in which the initial and final volts are fixed at 1 days, and the intermediate volts have 1 days in some years and 2 days in some years. At the beginning of the incubation period, it is called entering or alternating, and at the end of the incubation period, it is called appearing. The number of middling days depends on the time of middling, generally before July 28th, middling is 2 days, and middling is 1 days on July 29th.

It is stipulated in the almanac that "the number of volts will be counted on the third Geng day of the summer solstice", which means that "counting backwards from the date of the summer solstice" and starting to crouch on the third Geng day. The first Geng day after beginning of autumn was the first day of the last fall; The tenth day of the last lodging is the lodging.

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