What does it mean to enter nine?

Entering nine is entering nine cold days.

Enter, which means enter; Nine is the index of nine cold days, so entering nine is: entering nine cold days.

After the "winter solstice" of the traditional 24 solar terms, the weather has entered the coldest stage of the year, which is what people often say. There is a folk saying that "the cold is in March 9th and the heat is in dog days". When counting to nine, pay attention to cold and warmth, especially for the elderly and children, and don't catch a cold and get sick.