Great Cold (Dahan) is the last of the twenty-four solar terms. It occurs annually between January 20 and 21 in the Gregorian calendar, when the sun reaches the ecliptic longitude of 300°. Like Minor Cold, Great Cold is a solar term that expresses the degree of coldness in the weather; Great Cold means that the cold has reached its extreme. The Great Cold solar term falls during the “three nines” and “four nines” periods (the coldest stretches of winter), making it the most frigid time of the year.

In ancient China, descriptions of cold and warmth were largely made through climatic and phenological phenomena. The cold of the north was always accompanied by ice, and the ancients judged the coldness of a solar term based on the condition of the ice. The reason Great Cold is called “great” is that at this time the ice reaches its greatest thickness of the year, and its surface area is also at its maximum. The Yueling Qishier Hou Jijie (Collected Explanations of the Seventy-Two Pentads of the Monthly Ordinances) uses the phrase “water and marshes freeze solid to their core” to describe the climatic features of Great Cold. In the view of the ancients, Great Cold was colder than Minor Cold.

During the Great Cold season, the point of direct solar incidence is moving from the Tropic of Capricorn toward the equator, but cold air still frequently surges southward, and temperatures in the Northern Hemisphere remain bitterly cold. A folk saying goes: “During Minor Cold and Great Cold, it is cold even without wind,” indicating that the Great Cold solar term is one of the coldest periods of the year and also the period with the least rainfall. During Great Cold, the atmospheric circulation is relatively stable, with an adjustment cycle of approximately 20 days. In some regions of China, the period from Great Cold to Beginning of Spring is marked by many important folk customs, such as sweeping out the old and bringing in the new, making cured meats, offering sacrifices to the Kitchen God, and holding the Wei Ya (year-end feast) sacrifice. The Wei Ya sacrifice, also known as “zuo ya” or “zuo ya ji,” includes the folk custom of the whole family sitting together to “eat the Wei Ya meal” after the ceremony. The popular year-end “annual gathering” is a surviving custom of the Wei Ya sacrifice.