What does Taoism mean by the first day of September to the ninth day of September?

It means: For example, today is the first day of September, and the nine planets of Beidou are born (from the first day to the ninth day).

Astronomically, it is called the Big Dipper, but in Taoist culture, it is called the Big Dipper nine planets, "Big Dipper nine planets, see (now) two hidden stars". Those two stars are called cave stars and hidden stars respectively. This is the traditional respect for the Nine in ancient times. Beidou had a high status in ancient China, and Taoism changed it into nine planets.

Another way of saying this is that the belief in the Nine Emperors was originally a belief in Beidou, which is the oldest star worship in China tradition. According to the Taoist Beidou classic, the birthday of the Ninth Emperor of Beidou is from the first day of September to the ninth day of September every year. For nine days in a row, worshipping the stars of the Big Dipper can help people eliminate disasters and live longer.