Dashu occurs at celestial longitude 120°, around July 22-24. Climatologically the hottest period of the year for most of China; the "middle fu" of the three fu days typically falls here.
Tradition: drinking herbal teas to prevent heatstroke (清涼茶), eating "fu" lamb in some regions. Climate: extreme heat, frequent thunderstorms, peak typhoon season in coastal areas.
The first of this term's three pentads states that rotting grass turns into fireflies. Observers saw fireflies rising out of damp decaying vegetation and recorded grass becoming beetle. We now know the larvae pupate among that litter and emerge as adults, so nothing transmutes. The entry is worth keeping precisely because it shows what the seventy-two pentads are: a long observational record of what was seen and when, not a theory of why. Read as a field record and not as a claim about mechanism, the list becomes more useful, not less. The other two pentads are damp earth in muggy heat and heavy rains falling in season: ground steaming, air close, convection strong enough for frequent downpours. Taken together the three describe humidity and heat compounded, which is what separates Dashu from other hot stretches, since the discomfort is as much about moisture as about temperature. On the name, the Yueling Qishi'er Hou Jijie line of commentary stays consistent: shu is heat, heat divides into lesser and greater, the middle of the month takes the greater, and this is where the wave tops out.
Dashu is the median term of wei month, so it moves nothing, yet it is often treated as the earthiest stretch of the year. Wei is dry earth holding ji earth, ding fire and yi wood; by the time Dashu arrives most allocations of the hidden stems have ji earth in its proper seat, and the conversion of fire into earth is out in the open. The doctrine giving earth command of the eighteen days before each seasonal beginning likewise assigns the run from Dashu to Liqiu to earth. For a chart rooted in wei, settle first whether you are drawing on its fire or its earth, because the conclusions often point opposite ways.
Reading Dashu as the last heat before autumn does not hold: Liqiu is a calendrical marker, and the late-season heat Chinese calls the autumn tiger often comes afterwards. Customs such as summer mutton or cooling herbal teas also get taken for universal health rules. These are regional habits, and north and south sometimes do the opposite of one another, one school meeting heat with heat and another with cooling. Both have long belonged to the tradition. They read best as records of custom, not as a single prescription.
Set beside Xiaoshu and Liqiu, Dashu closes the summer instead of opening its climax. Xiaoshu starts the fu days, Dashu usually lands inside the middle fu, and one more fu period still follows the beginning of autumn, which is why the saying about a fu remaining after autumn arrives is not a contradiction: fu periods are counted from geng days and do not end when the season's name changes. Against the solstice the contrast is clean. Xiazhi is the maximum of sunlight, Dashu the maximum of stored heat, and the month between them is the lag.
Dashu has a second job in the calendar: it is one of the median terms that decide intercalation. A lunar month containing no median term becomes a leap month, and Dashu is the one that settles whether a year carries a leap sixth month. The middle fu, meanwhile, lasts ten or twenty days depending on how many geng days fall between the solstice and the beginning of autumn, so which day of the middle fu Dashu lands on changes from year to year. Printed calendars agree on this, while popular shortcuts assuming a fixed count sometimes do not.
Not literally. Observers saw fireflies emerging from damp decaying grass and recorded the grass as becoming the insect. In fact firefly larvae pupate among that litter and damp soil before emerging as adults, so no transformation of substance occurs. The value of the pentad lies in its accuracy as observation: the timing and the phenomenon were recorded correctly while the explanation reflected the knowledge of the period. The whole set of seventy-two pentads reads best on those terms.
Dashu names the hottest stretch rather than a single hottest day. It marks 120 degrees of celestial longitude, the point the traditional calendar treats as the crest of summer heat, and in most years the middle fu falls within this window. Actual maxima depend on the year and the region: some places peak before Dashu and others after the beginning of autumn. The calendar supplies a scale, not a record of any particular year's temperatures.
No. Dashu is one of the twelve median terms and sits in the middle of wei month; the pillar changes at the sectional terms Xiaoshu and Liqiu instead. It still features in chart reading. Wei is dry earth, and by around Dashu most allocations of the hidden stems place ji earth in its proper seat, so the conversion of fire into earth is at its most visible. The period is often described as one of the earthiest of the year.
Because the fu days are counted from stems and branches rather than ending with the season's name. The last fu is fixed at the first geng day after Liqiu and runs ten days, so a hot period extending past the start of autumn is built into the rule, not an exception. The first fu starts on the third geng day after the solstice and the middle fu lasts ten or twenty days, which can carry the whole block to forty days.
The solstice is the maximum of sunlight, with the longest day and the highest noon sun. Dashu is the maximum of accumulated heat, when the ground has absorbed more than it has released for weeks on end. The roughly one-month gap between them is exactly that lag. Both are median terms, in wu month and wei month respectively, and neither changes the Bazi month pillar, but one marks a turn in yang energy and the other the crest of the heat.