Daxue occurs at celestial longitude 255°, around December 6-8. Heavier snowfall and persistent cold settle across northern China; deep winter truly begins.
Tradition: hot pot dinners gather families; salted fish (腌鱼) preparation. Climate: rivers and lakes start freezing in the north; southern regions enter their cold rainy season.
The classical commentary glosses Daxue (大雪 Dà Xuě) as greater in the sense of full strength, the point from which snow becomes abundant, set deliberately against the not-yet-full-strength of Xiaoxue. Its three pentads repay attention: the hedan bird falls silent, tigers begin to mate, and the liting plant puts out shoots. The hedan was the legendary bird said to cry through the night for dawn, so its silence pictures yin at its densest — yet mating tigers and sprouting shoots are signs of life stirring. Placing stillness and stirring side by side in one term is deliberate foreshadowing, not inconsistency. The mating tiger and the sprouting plant record living things that register the return of yang before a thermometer would. Liting is generally taken to be a cold-hardy narrow-leaved herb that puts out new shoots in the cold, and tigers mating in midwinter were likewise read as evidence of yang first stirring. In the fields, rivers begin to freeze in the north around Daxue and the soil hardens to depth, so outdoor work largely stops and the season turns to composting, repairing tools, cellaring and maintaining waterworks — one of the few stretches of the year given to tidying rather than producing.
Daxue is a sectional term, so the month pillar turns as it is crossed: the month branch goes from Hai (亥) to Zi (子). Zi carries the simplest hidden-stem content of any branch, Gui (癸) water alone. Among the four cardinal branches, Zi, Mao and You each hold a single stem, while Wu additionally holds Ji earth. A pure branch shows its nature plainly: water in the Zi month is unmixed, extreme when strong and bitter when cold, without the thread of Jia wood that Hai still preserved. That is the substantive change Daxue brings.
The name is the easiest thing here to over-read as a promise of heavy snow. Daxue means only that the sun has reached 255° of celestial longitude, usually 6–8 December. In many years no snow falls during the term at all, while genuine blizzards arrive around Xiaohan, Dahan or even after Lichun. A second frequent error is equating Daxue with the whole eleventh lunar month; Daxue is only the opening of the Zi month, which continues until Xiaohan.
Zi's relationships with the other branches come up constantly in winter charts. Zi clashes with Wu (午), the most direct of the six oppositions. Zi combines with Chou (丑) toward earth, though because both are cold and damp, practitioners often judge the combination to bind without transforming. Zi, Hai and Chou together assemble the northern water direction, linking the three winter months. In the Shen–Zi–Chen water trinity, Zi sits at the centre as the peak. Zi and Mao also form the so-called discourteous punishment. Checking which of these are activated says more than simply noting that water is strong.
What a Zi-month chart needs is one of the sharpest disagreements in the tradition. The climatic-balancing school holds that winter water is cold to an extreme and cannot be warmed without fire, putting Bing (丙) fire first. The strength-and-structure school insists on judging the day master's strength and the integrity of the chart's pattern first, treating climatic balance as a final adjustment. Faced with the same chart the two often reach opposite conclusions. Most practitioners work through both lines before deciding, instead of committing to one lineage.
Daxue sits between Xiaoxue and the solstice in a delicate position: yin is still accumulating, but the turn is only a fortnight away. The twelve sovereign hexagrams assign the Zi month to Fu (復), one yang line born beneath five yin, and the Zi month begins at Daxue, so strictly the birth of that yang line falls somewhere in the span between Daxue and the solstice, with the solstice merely its clearest astronomical marker. Read that way, the mating tigers and sprouting shoots in Daxue's pentads stop looking out of place.
Often it does not. Daxue only marks the sun reaching 255° of celestial longitude, around 6–8 December, and is not a forecast. The name records a general impression from the historical Yellow River valley that snowfall grows heavier around then. In practice the largest snowfalls across much of China tend to come during Xiaohan or Dahan, or even after Lichun, so the sequence of names does not track observed snow depth.
Daxue is a sectional term: crossing it turns the month branch from Hai to Zi and changes the month pillar. Dongzhi is a mid-term, sitting at the midpoint of the Zi month and changing no pillar. They are about a fortnight apart. In meaning, Daxue says yin is still accumulating, while the solstice marks daylight turning from shortening to lengthening. Since the twelve sovereign hexagrams assign the entire Zi month to Fu, the two can be read as the front and back of a single turning.
Because in the yin-yang framework yin at its extreme necessarily gives rise to yang, and the earliest signs of that turn were thought to show in animal behaviour rather than in temperature. Daxue opens the Zi month, only two weeks from the solstice and its return of the single yang. Recording mating tigers and sprouting shoots here marks the approach of that turn, and the pairing of silence and stirring in one term is intentional.
They vary a great deal by region. Northern households cellar cabbage and root vegetables, finish winter irrigation and repair tools. In the south, Fujian and Taiwan, the cold dry air is used for curing pork and air-drying fish. Gathering around a stove or a hotpot is also common, echoing the themes of sealing in and drawing together. These are local production and food traditions, not any standard observance.