How do we describe 九九 in english
This Beijing Tourism page has an English article about the 9 nine-day periods:

Nines of Winter in China
People in China talk about winter coming in nine periods and each period is 9 days long, which begins from the Winter Solstice. After 9 nine-day periods the weather becomes warmer and spring will be on its way.
The 3rd nine-day period is called ‘Sanjiu’ in Chinese, which refers to the 19th-27th days after the day of the Winter Solstice. There is an old saying in China that goes, &quot;The days of the Sanjiu period are the coldest days.&quot;
There is a famous folk song about the Nines of Winter (called ‘Shujiu’) recording changes in the weather:
1st nine days, 2nd nine days, don’t take hands out of your pockets;
3rd nine days, 4th nine days, you can walk on ice;
5th nine days, 6th nine days, willows at the river’s edge start to sprout;
7th nine days, ice dissolves and water flows in the river;
8th nine days, wild geese fly back to northern areas;
9th nine days and the following days, farm cattle start to work in the field.