Dahan occurs at celestial longitude 300°, around January 20-21. The 24th and final solar term — typically the coldest. After this, Lichun begins a new annual cycle.
Tradition: preparing for Lunar New Year (cleaning, shopping, making food). Climate: extreme cold in the north; spring is hinting at southern China.
Dahan (大寒 Dà Hán) carries three pentads: hens begin to brood, birds of prey hunt with fierce speed, and the ice on waters and marshes freezes solid to its centre. The first two are biological, the third topographic. Among the twelve sovereign hexagrams the Chou month takes Lin (臨, Approach), whose lower two lines are already yang — the single yang line of Return has grown to two. Still buried, but no longer tentative. Maximum cold set alongside growing yang is the basic structure of this term.
Coming last among the twenty-four invites the assumption that Dahan must be the coldest. In northern China, Xiaohan has the lower average temperature in most years. Final position is an artefact of how the sequence was arranged, not a verdict about climate extremes. Treating Dahan as the end of the lunar year does not hold either; lunar New Year's Eve is set by the new moon and has no fixed relation to it, sometimes falling inside Dahan and sometimes after Lichun.
The eighteen days or so after Dahan are what the tradition calls the period when earth holds office. Each season is credited with eighteen earth-governed days, placed immediately before one of the four beginnings. The stretch before Lichun belongs to Chou earth, which holds authority until the crossing into the Yin (寅) wood month. Two things follow in practice: the later part of the Chou month has a different force structure from its opening, and these eighteen days act as a buffer between water and wood, since the tradition does not let two elements meet directly but hands over through earth. Dahan is a mid-term at the centre of the Chou month and changes no pillar. What deserves attention is the step after it: at Lichun the year pillar and the month pillar turn together, the only seam in the system where two pillars move at once. Anyone born between Dahan and Lichun still carries the previous year's year pillar, and this holds even if the lunar New Year has already passed. It is the easiest stretch of the calendar to get wrong, and it must be settled from the crossing time, never inferred from the lunar or Gregorian year.
When the annual cycle turns over is a genuine point of divergence, and it comes to a head right after Dahan. Bazi conventionally uses Lichun. Mainstream Ziwei Doushu takes the first day of the first lunar month as the start of the annual cycle, though some lineages compromise. Older calendars began the year at the solstice altogether. The three answers belong to chart calculation, folk calendar and historical astronomy respectively, and each is defensible on its own terms. Checking which convention a tool uses is more useful than arguing over which is correct.
Dahan stands opposite Dashu on the ecliptic, as Chou stands opposite Wei among the branches, and the two clash. Tradition reads this as a clash between storehouses: Chou holds Xin metal and Wei holds Yi wood, so what each stores is disturbed when they meet. In the sequence, Dahan follows Xiaohan and opens onto Lichun, making it the hinge between ending and beginning. It is nominally the twenty-fourth term, yet nothing separates it from Lichun, and the last eighteen days of winter are already preparation for the next round.
Waters freezing solid to the centre describes the heaviest ice of the year, and it was around Dahan that ice was traditionally cut for storage in icehouses, being dense and slow to melt. Households turn fully to New Year preparation: sweeping out dust, buying provisions, making festival food, honouring the stove. Preparing for a new year during the coldest weeks is not a contradiction — field work has stopped, labour shifts to the home, and with Lichun close the whole seasonal round is switching from storing to starting. The cycle closes here, but the closure is a pivot, not an end.
Not necessarily. It comes last in the sequence, but across most of northern China the average temperature during Xiaohan is lower, while in the Yangtze valley and the south the two are close. The name Greater Cold comes from the convention of pairing it with Lesser Cold, not from a ranking of measured temperatures. Actual minima also depend on the track of cold surges, distance from the sea and altitude, and vary by year and place.
Not yet. Dahan is a mid-term and changes neither the month pillar nor the year pillar. The Bazi year turns at Lichun, so anyone born between Dahan and Lichun still carries the previous year's year pillar. Note especially that this holds even after lunar New Year has passed: the lunar year and the Bazi year pillar are two separate boundaries.
No, and there is no fixed relation between them. Dahan is set by solar longitude and lands reliably on 20 or 21 January, while New Year's Eve is set by the new moon and drifts, falling anywhere from late January to the middle of February. Sometimes it lands inside the Dahan term and sometimes only after Lichun, depending entirely on where the new moons fall that year.
Not necessarily, and this is a common point of divergence. Bazi conventionally uses the crossing of Lichun as the year boundary. Mainstream Ziwei Doushu reads the annual cycle from the first day of the first lunar month, though some lineages use Lichun as well. Between Dahan and Lichun, and again between Lichun and lunar New Year, the two can report different annual pillars. It is worth confirming which convention any chart tool uses.