What's the meaning of "nine" in Jiujiu Song?

There is a saying "Jiujiu" in the China lunar calendar, which is used to calculate the seasons. Starting from the winter solstice, the first nine days are called "September 19th". The second nine days are called "29", and so on until "99". That is, the ninth nine days, winter has passed and spring has arrived.

This "99" theory. As early as the Southern and Northern Dynasties. At this time, the number of solstice days in winter has reached 998 1, and the cold days are over. This 99 song originated in the Song Dynasty and was very popular in the Ming Dynasty, but when it was popular among the people, it had different versions, different contents and details.

Extended data:

Jiujiuge (Beijing)

1929 did not shoot; Walking on the ice in 3949;

5969 see willows along the river; 79 rivers open 89 geese;

99 plus 19, plowing cattle everywhere. ?

Jiujiuge (Changsha)

On the ninth day of the ninth lunar month, don't shoot when you meet (put your hands in your sleeves or pockets);

3927, hanging a pen in front of the eaves (icicles);

In 4936, pedestrians spent the night (going home for the New Year);

In 5945, poor Han people danced in front of the stage (praising spring and sending the god of wealth);

In 6954, the withered branches made tender thorns;

7963, undressing on the sidewalk;

In 8972, Hippies (frogs) burped in the field;

998 1, take off the hemp fiber and put on the hat.

Nine Jiu Ge (Jiangsu Changwu)

Don't shoot when you meet before September 29; In 3949, the cable was shivering with cold;

Dancing in the streets of poor Han people in May, 945; 6954 mosquitoes and flies squeak;

7963 pedestrian clothing list; 8972 stepping on mud barefoot;

Bloom added green leaves in 998 1 year.