The third and fourth Geng days after the summer solstice are the beginning dates of the first (head) and the middle, respectively, and the first Geng day after beginning of autumn is the first day of the last.
According to the traditional calculation method in China, the third and fourth Geng days after the summer solstice are the starting dates of the first (head) and the middle, respectively, and the first Geng day after beginning of autumn is the first day of the last. Because the interval between each Geng day is 10 days, the stipulated time for initial and final lodging is 10 days. And because the third Geng day (first fall) after the summer solstice solar terms appears sooner or later, the number of days in the fall is long or short, which may be 10 or 20 days.
Sanfu diet culture
As the saying goes, "cold has three or nine, and heat has three volts." During the dog days, on the one hand, the ancients used cool drinks to escape the heat, on the other hand, they held corresponding customs activities, such as "drinking to avoid evil". Over time, a unique food custom culture has been formed.
China is a vast country with different customs in different dynasties and regions. For example, in the Northern Song Dynasty, the summer diet was a "green purse", and the "green lotus" was a lotus leaf, which had the effect of clearing away the heat.
On dog days in Taizhou, Zhejiang, the elderly have the custom of eating chicken porridge, which is called "Buyang". In a word, the origin of the three-dog food custom in China is related to the "Fu La Festival" written in the Han Dynasty, which was originally an activity to celebrate agriculture. This also reflects the richness and variety of China's food customs culture from one side.
Reference to the above content: Baidu Encyclopedia-Sanfu