TL;DR: Đại Thử is the 12th of the 24 solar terms and the mid-term of the sixth lunar month. It begins when the Sun reaches 120 degrees of ecliptic longitude, on 22 or 23 July, and lasts until Lập Thu begins on 7 or 8 August. True to its name, "great heat", it is regarded as the hottest stretch of the year in the Northern Hemisphere. In Vietnam the term has no festival of its own; the book Tuế thời quảng ký records its three pentads: rotting grass turning into fireflies, damp muggy soil, and heavy seasonal rain. In Bát Tự, Đại Thử does not change the month, so a person born in this term still has the Mùi month.
What is Đại Thử (Major Heat)?
Đại Thử (大暑) is the 12th solar term in the East Asian system of 24 solar terms. The character 大 means great and 暑 means heat, so the name literally means "great heat". The term comes right after Tiểu Thử and before Lập Thu, and it is the last mid-term (trung khí) of summer.
In old books, Đại Thử is the mid-term of the sixth lunar month. The book Tuế thời quảng ký (歲時廣記) quotes the Tam thống lịch (三統厯): "小暑為六月節大暑為六月中氣小暑大暑者就極熱之中分為小大月初為小月半為大"[3], meaning Tiểu Thử is the term of the sixth month and Đại Thử is its mid-term; the two names split the peak heat into small and great, the start of the month being small and the middle being great. Because it is a mid-term rather than a term, Đại Thử does not open a new month when months are divided by the solar terms.
When does Đại Thử begin?
Đại Thử begins when the Sun reaches 120 degrees of ecliptic longitude and ends when it reaches 135 degrees, the moment Lập Thu begins. In Vietnam time, from 1900 to 2100 Đại Thử begins on 22 or 23 July; in some years between 1902 and 1947 it fell on 24 July. Lập Thu usually begins on 7 or 8 August, so the last day of Đại Thử is usually 6 or 7 August. Start times for the coming years:
| Year | Start date of Đại Thử | Time of the term change (Vietnam time) |
|---|---|---|
| 2025 | 22 July 2025 | 20:27 |
| 2026 | 23 July 2026 | 02:12 |
| 2027 | 23 July 2027 | 07:58 |
| 2028 | 22 July 2028 | 13:45 |
| 2029 | 22 July 2029 | 19:32 |
| 2030 | 23 July 2030 | 01:21 |
The book Mệnh lý tham nguyên by Viên Thụ San quotes the classic: "五日谓之候，三候谓之气，六气谓之时，四时谓之岁"[2], meaning five days make one hậu (pentad), three hậu make one khí, six khí make one season and four seasons make one year; the same passage states "三候成一气，即十五日也"[2], three hậu make one khí, that is fifteen days. This is why each solar term lasts about 15 days.
The table below shows where Đại Thử sits among the summer solar terms, with Gregorian dates for the years 2000 to 2050:
| Solar term | Position in the year | Solar longitude | Gregorian date |
|---|---|---|---|
| Lập Hạ | 7th term | 45 degrees | 5 or 6 May |
| Tiểu Mãn | 8th term | 60 degrees | 20 or 21 May |
| Mang Chủng | 9th term | 75 degrees | 5 or 6 June |
| Hạ Chí | 10th term | 90 degrees | 20 to 22 June, mostly 21 |
| Tiểu Thử | 11th term | 105 degrees | 6 or 7 July |
| Đại Thử | 12th term | 120 degrees | 22 or 23 July |
| Lập Thu | 13th term | 135 degrees | 7 or 8 August |
Đại Thử falls at the end of summer in the solar-term calendar, about a month after Hạ Chí. Hạ Chí is when the Sun stands highest in the Northern Hemisphere, but land and water need more time to store heat, so in many places the heat peak comes later, around the Đại Thử period.
Why is it called "great"? How old books explained it
The book Mệnh lý tham nguyên, in its section on the 24 solar terms (二十四节气), quotes Vạn Dục Ngô (万育吾) on the names of the terms: "六月节小暑，六月中大暑，夏至后，暑已盛，不当又谓之小"[2], meaning Tiểu Thử is the term of the sixth month and Đại Thử its mid-term; after Hạ Chí the heat is already strong, so it seems it should no longer be called "small". He answers with the I Ching: "寒往则暑来，暑往则寒来。寒暑相推，而岁成焉"[2], when cold goes heat comes, when heat goes cold comes, and cold and heat pushing each other make the year. In his view the whole first half of the year can be called hot and the whole second half cold, so "small heat" and "great heat" are only words within the first half; the middle of the sixth month is the peak of the heat, hence "great", and before the peak it is still "small".
This explanation rests on the yearly cycle of heat and cold, yin and yang, not on measured temperature.
What are the three pentads of Đại Thử according to old books?
The book Tuế thời quảng ký, in its section on the last month of summer (季夏月), quotes the Hiếu kinh vĩ (孝經緯): "夏至後十五日斗指丁為小暑後十五日斗指未為大暑"[1], meaning that fifteen days after Hạ Chí the handle of the Northern Dipper points to Đinh, which is Tiểu Thử, and fifteen days later it points to Mùi, which is Đại Thử. The same section quotes the Chu thư thời huấn (周書時訓) on the three pentads (hậu) of the term: "大暑之日腐草為螢後五日土潤溽暑後五日大雨時行"[1]. That is, on the day of Đại Thử rotting grass turns into fireflies; five days later the soil is damp and the air muggy; five days after that heavy seasonal rains arrive.
These three pentads record how the ancients observed nature in the hot, humid period. "Rotting grass turning into fireflies" is an old understanding; fireflies are insects that hatch from larvae and are seen most in summer, not grass transformed.
The same section quotes the Nguyệt lệnh (月令): "季夏行春令則穀實鮮落國多風欬民乃遷徙行秋令則丘隰水潦禾稼不熟乃多女災行冬令則風寒不時鷹隼蚤鷙四鄙入保"[1]. This passage says that if, in the last month of summer, the ordinances of spring, autumn or winter were carried out, matching disasters would follow, such as grain dropping early, flooded fields, crops failing to ripen and unseasonal cold winds. It is a political and philosophical admonition of old books, not a weather forecast.
What folk customs are there around the Đại Thử term?
In Vietnam, Đại Thử has no festival of its own like Tết Nguyên Đán or Tết Đoan Ngọ. It usually falls in the sixth lunar month and is known mainly as a marker printed in the solar-term calendar.
In folk belief, Đại Thử is regarded as the hottest time of the year, as its name "great heat" suggests.
Tuế thời quảng ký is a Chinese book of the Southern Song dynasty; what it records about the sixth month is Chinese material, not Vietnamese custom.
How is the Đại Thử term used in divination systems?
In feng shui and East Asian divination systems, the 24 solar terms are markers for choosing days, placing stars and determining timing.
In Bát Tự, the month pillar changes only at the 12 terms; Đại Thử is a mid-term, so it does not change the month. The Mùi month runs from the moment Tiểu Thử begins until just before Lập Thu begins, so a person born during Đại Thử still has the Mùi month. The book Sử dụng la bàn by Thanh Thủy has a table matching the 24 mountains with the 24 solar terms on the Thái âm đáo sơn plate, in which Đại Thử corresponds to the Mùi mountain[4]. This is a lookup rule of the discipline, not calendar data that can be verified.
How does Đại Thử differ from other terms?
To position Đại Thử, it can be compared with neighbouring terms:
| Solar term | Meaning of the name | Weather characteristics |
|---|---|---|
| Tiểu Thử | Small heat | Intense sunshine begins, not yet at its extreme |
| Đại Thử | Great heat | Most intense sunshine, heavy seasonal rains |
| Lập Thu | Beginning of autumn | Sunshine gradually softens, ngâu rains occur |
| Xử Thử | End of heat | Heat remains but has lessened |
| Bạch Lộ | White dew | Cooler, dew appears |
This comparison shows that Đại Thử is the peak of the warming sequence in summer. After Đại Thử, the following terms mark the gradual decline of the heat: Lập Thu opens autumn, Xử Thử ends the hot period, and Bạch Lộ signals dew forming at night. This is the natural cycle of the Northern Hemisphere, not a rule that applies to every climate zone.
How should the Đại Thử term be used in daily life?
Nhật Nguyệt's suggestion: the start date of Đại Thử shifts between 22 and 23 July, so to know the exact moment this year, check the time of the term change in Vietnam time, as in the table above, rather than remembering a fixed date. If you plan to repair a house or a roof in this period, keep an eye on rain and storm forecasts.
What do people often ask about the Đại Thử term?
Is Đại Thử the hottest day of the year?
In folk understanding, Đại Thử is regarded as the peak of the hot season. However, actual temperature also depends on latitude, terrain, monsoons and many other climatic factors. In some regions, the heat peak may fall at Tiểu Thử or a few days after Đại Thử. Therefore, Đại Thử should not be treated as absolutely the hottest day everywhere.
Why does the starting date of Đại Thử change between years?
Because the solar term calendar follows the actual position of the Sun on the ecliptic, not a fixed calendar. A Gregorian year has 365 or 366 days and does not match exactly the time the Sun takes to return to the same point on the ecliptic, so the date of the term change moves back and forth between 22 and 23 July.
Does the Đại Thử term affect the choosing of auspicious and inauspicious days?
In East Asian divination systems, solar terms are a factor used when choosing days, placing stars, and determining timing. However, this is a convention of the discipline, with no scientific evidence confirming a link between solar terms and human fortune. If you use it for cultural reference, you should clearly know that this is a traditional belief, not a verified conclusion.
