Dàshǔ

Dashu, Major Heat, is the twelfth solar term, at the sun's longitude of 120 degrees, around the twenty-third of July. It is a principal (zhongqi) term and generally the hottest, most humid stretch of the year, the height of summer thunderstorms and the deep dog days. In the farming almanac it is a period of vigilance against heat, drought, and flood, with the crops in their most demanding phase of growth. Traditional customs address the heat through cooling foods and, in some regions, tonic dishes eaten to fortify the body. Being a solar marker, it falls near the same Gregorian date each year. These are the observances of the East Asian agricultural tradition, resting on the division of the tropical year into twenty-four solar terms of fifteen degrees each along the ecliptic.

Dashu, Major Heat, falls at 120 degrees, around 22 or 23 July, and is a qi, or major, term, traditionally regarded as the hottest of the twenty-four terms and the peak of the summer heat sequence that began with Xiaoshu two weeks earlier. It falls within the sanfu period, the traditional three-stage reckoning of the year's hottest weeks, calculated from the sexagenary day cycle rather than from the solar terms directly, during which several regional folk customs prescribe specific foods thought to counter the heat, including mung bean soup and, in some areas, a light rice porridge, reflecting a broader traditional Chinese medical framework in which diet is adjusted to the perceived qualities of each season.

The twenty-four solar terms divide the ecliptic into equal arcs of fifteen degrees, and this one falls at 120 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 system travelled with the Chinese calendar. Japan, Korea and Vietnam all adopted the twenty-four terms and kept their own readings of the same names, and the terms remain in use in each. In 2016 the twenty-four solar terms were inscribed on the UNESCO Representative List of the Intangible Cultural Heritage of Humanity, on the ground that they remain a living framework for agricultural and everyday practice rather than an obsolete scheme.

Dashu, major heat, is the last of the summer terms and the hottest point of the year in the climate the system describes. It falls within the sanfu period counted from the summer solstice, so the two ways of naming the height of summer, by solar term and by the fu count, overlap without coinciding. That kind of overlap is common in the Chinese calendar, which carries several independent cycles at once and does not reduce them to each other.