‘Sān jiǔ’ (三九) refers to the coldest period of winter in traditional Chinese seasonal reckoning, specifically the third nine-day segment after the Winter Solstice. The term originates from the ‘Nine Nines’ (数九) custom, where each nine-day period marks a stage in winter’s progression: the first nine days is ‘yī jiǔ’, the second ‘èr jiǔ’, and so on—culminating in ‘sān jiǔ’, widely regarded as the harshest stretch of cold weather. Though not an official calendar date, it typically falls around mid-January and is deeply embedded in folk sayings, health practices, and regional customs.

The characters literally mean ‘three’ (三) and ‘nine’ (九), reflecting its position in the sequential count. It functions as a fixed noun phrase and is rarely used outside meteorological or cultural contexts—often appearing in weather reports, traditional medicine advice (e.g., ‘sān jiǔ tǐng’ — ‘Three-Nine strengthening’ acupuncture), or colloquial warnings like ‘Sān jiǔ tiānqì zhēn lěng!’ (The Sān Jiǔ weather is truly freezing!).