What does the cold in winter mean?

The cold of March 9 in winter refers to:

"March 9th" refers to the third "nine days" after the winter solstice, that is, the 19th to 27th days after the winter solstice. There is a saying in the China lunar calendar called "Jiujiu", which is used to calculate the seasons. Generally, it is the coldest in "Sanjiu". Therefore, it is called the "March 9 Great Cold". The solstice of winter begins, and it will be very cold on March 9 after 27 days.

The calculation method is from the solstice of winter (counting from the solstice of winter, which means the beginning of cold), every nine days are called a nine, the first nine days are called a nine, the second nine days are called a 29, and so on until the ninth nine days, when the number reaches 998 1 day. At this time, winter has all passed and spring has arrived.