"March 9" is the lowest temperature solar term in a year, marking the beginning of the coldest day in a year. As the saying goes, a slight cold is not as good as a severe cold, and the coldest is in "Sanjiu".
"April 9" means entering the cold weather from winter to the future, and the fourth nine days are called "April 9".
From counting to nine, it really entered the coldest winter of the year. That is, from the winter solstice to the Sunday, a "nine" is counted every nine days until the "nine-nine" of 81 days, which roughly includes the three-month winter in the Gregorian calendar. Forty-nine days is four nine days from the winter solstice, that is, thirty-six days, lasting nine days. Among them, Sanjiu and Jiu Si are the coldest.