The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 270 degrees of the sun's apparent longitude. It is a major term, a zhongqi, one of the twelve that fall in the even positions of the twenty-four. The major terms are the ones that anchor the lunar months, and they alternate strictly with the sectional terms, so a zhong is always followed by a jie fifteen degrees later.
The twelve major terms are what make the Chinese calendar work as a lunisolar one. Each ordinary lunar month is expected to contain exactly one major term, and the month takes its number from the term it contains, so the eleventh month is by definition the one holding the winter solstice. When a lunar month falls entirely between two major terms and contains none, it is declared intercalary and repeats the number of the month before it, which is how a thirteenth month is inserted without any fixed rule about when.
Dongzhi is the winter solstice and the pivot of the entire calendar, because the eleventh lunar month is defined as the month containing it. Every other month number follows from that anchor, and so does the placement of any intercalary month. The day is also a substantial festival, marked in different regions by tangyuan or by dumplings, and the shujiu, a count of nine periods of nine days through the depth of winter, is reckoned from it.