What do the solar terms "Sanjiu" and "Jiu Si" mean?

"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 "Jiujiu" in the China lunar calendar, which is used to calculate the seasons. 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 passed and spring has arrived.

Generally, it is the coldest in "Sanjiu". The "third ninth day after the winter solstice" mentioned by friends upstairs refers to "March 9", which is also the day when March 9 and April 9 intersect, not "March 9".