In ancient China, the method of "recording the date by branches" was popular, and 60 groups of different names matched with 10 heavenly stems and 12 earthly branches were used to record the date for recycling. The day with the word "Geng" is called "Geng Day". The word "Geng" in Geng Day is the seventh word in "A, B, C, D, E, Ji, G, Xin, Ren, Gui" 10 day, and it is repeated every 10 day. When there are four Geng days between the summer solstice and the beginning of autumn, the middle part is 10 days. If the fifth Geng Day is before beginning of autumn, it will take 20 days to get to Zhongfu, commonly known as the second Zhongfu; If it is after beginning of autumn, the median value is 10 days. It's sultry and humid in dog days, so we should pay attention to health preservation, eat more food to clear away heat and relieve summer heat, and don't drink cold drinks to relieve summer heat when it's hot in dog days, because the yang in dog days floats on the body surface, and all the internal organs are cold. So eating too much cold food is not good for your health. Generally, it is hot in the north at first, and hot in the middle and south. "Fu" can be called "long summer", and "San Fu" is the general name of the first, middle and last fu, which is the hottest season in a year, appearing from mid-July to mid-August in the solar calendar every year. During the three-volt period, the temperature is high, the air pressure is low, the humidity is high, and the wind speed is low, among which the hottest is in the middle. "V" means hiding in a hot place, which has a warning effect. During the three-volt period, the temperature is high, the air pressure is low, the humidity is high, and the wind speed is low, among which the hottest is in the middle.