Why is it the third night in September?

In 82 1 year, Bai Juyi wrote the poem Ode to Mujiang. "the third day of September" coincides with the cold dew festival of that year. Why do you say "that year"? Because the time of the cold dew festival is different every year. In the ancient calendar of China, the solar term "cold dew" began when the sun reached the longitude of the Yellow River 195 degrees, so the specific date was not fixed. The third day of August 21September is not an ordinary day. The scene described in Bai Juyi's poem "Dew is like a pearl and the moon is like a bow" coincides with the phenological characteristics of the Cold Dew Festival.