the coldest time of winter, specifically the nine periods of nine days each following the winter solstice

First part of the two-part allegorical saying (xiehouyu), the second part being [[点滴在心头|點滴在心頭|dian3 di1 zai4 xin1 tou2]]; it literally means drinking cold water in winter and figuratively refers to remembering every bit of past hardships vividly