TL;DR: Tiểu Thử (小暑) is the 11th of the 24 solar terms, beginning when the Sun reaches ecliptic longitude 105 degrees. In Vietnam time it usually begins on July 7, in some years on July 6 or 8; in 2026 it began at 8:49 on July 7, and in 2027 it begins at 14:34 on July 7. It is the term that opens the Mùi month, so the month pillar in Bát Tự changes at that moment. The Vietnamese have no ritual of their own for this term; Tuế thời quảng ký records only its calendrical statements and three pentads.
What is Tiểu Thử?
Tiểu Thử is the 11th of the 24 solar terms of the East Asian calendar, coming right after Hạ Chí and before Đại Thử. Its name is made of two Chinese characters: 小 (tiểu, small) and 暑 (thử, heat), literally "minor heat". In the classical explanation, the term marks the stretch when the heat has become clear but has not yet reached its peak, so it is called "tiểu" to set it apart from "đại" in the next term.
Tiểu Thử is one of the 24 solar terms of the solar year, not a fixed lunar date. Each year it falls on a slightly different Gregorian date, between July 6 and 8, depending on when the Sun reaches 105 degrees of ecliptic longitude.
In calendrical terms, Tiểu Thử begins when the Sun reaches ecliptic longitude 105 degrees. It is the term that opens the Mùi month, the sixth month counted by solar terms; Đại Thử, which follows, is the middle qi of that month.
Calendrical mechanism: why is Tiểu Thử not fixed to one day?
The 24 solar terms divide the ecliptic into 24 parts of 15 degrees each; Tiểu Thử corresponds to the 105-degree point. A Gregorian year has 365 or 366 days, while the Sun takes about 365.24 days to return to the same longitude, so the moment of the term moves about 6 hours later each year and is pulled back after a leap year. That is why the Gregorian date of Tiểu Thử shifts between July 6, 7 and 8.
The book Mệnh lý tham nguyên states: "六月节小暑，六月中大暑", meaning that the term opening the sixth month is Tiểu Thử and the qi in the middle of the sixth month is Đại Thử[1]. It asks why the term is still called "minor" when the heat is already strong after Hạ Chí, and answers: "六月中大暑之极，故谓之大，然则未至于大，则犹为小也", meaning that the middle of the sixth month is the peak of the heat and so is called major, while before that point it is still minor[1]. The same book quotes: "五日谓之候，三候谓之气", that is, five days make one pentad and three pentads make one qi, about fifteen days[1].
In reality the terms are not equal in length, because the Earth moves faster or slower along its orbit. Tiểu Thử falls when the Earth is near its farthest point from the Sun and moves slowly, so it lasts about 15 days and 17 hours.
On what day does Tiểu Thử begin this year and in the coming years?
In Vietnam time, Tiểu Thử usually begins on July 7: from 1900 to 2100, 117 years fall on the 7th. Thirty-nine years fall on the 8th, all before 1980; from 2012 onward, 45 years fall on the 6th, such as 2024 and 2028. The term lasts until Đại Thử begins, usually on July 22 or 23.
Start times of Tiểu Thử in the coming years, in Vietnam time (UTC+7):
| Year | Gregorian date | Start time | Lunar date |
|---|---|---|---|
| 2025 | July 7 | 3:03 | 13th day of the 6th month |
| 2026 | July 7 | 8:49 | 23rd day of the 5th month |
| 2027 | July 7 | 14:34 | 4th day of the 6th month |
| 2028 | July 6 | 20:21 | 14th day of the leap 5th month |
| 2029 | July 7 | 2:08 | 26th day of the 5th month |
| 2030 | July 7 | 7:57 | 7th day of the 6th month |
Is Tiểu Thử the hottest day of the year?
No. In the naming of the classical books, Tiểu Thử is when the heat is clear but not yet at its peak, and the peak is Đại Thử, which begins on July 22 or 23. The actual hottest day in any place depends on the local climate and the year, and does not coincide with a fixed term.
What customs are associated with Tiểu Thử?
The Vietnamese have no ritual of their own for Tiểu Thử. The term is usually known only through printed calendars and in divination schools such as Tử Bình and Kỳ Môn Độn Giáp.
What does Tuế thời quảng ký record about Tiểu Thử?
Tuế thời quảng ký (歲時廣記) by Trần Nguyên Tịnh of the Southern Song, in the four-volume Tứ khố toàn thư edition, arranges its records by the four seasons and takes the Nguyệt lệnh, the Hiếu kinh vĩ and the Tam thống lịch as its framework[3]. In its entry for the last month of summer (季夏月), it records calendrical statements and the three pentads of Tiểu Thử, not any custom of its own. It quotes the Hiếu kinh vĩ: "夏至後十五日斗指丁為小暑", meaning that fifteen days after Hạ Chí, when the handle of the Northern Dipper points to the Đinh direction, it is Tiểu Thử[3]. It quotes the Tam thống lịch: "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", meaning that Tiểu Thử is the term of the sixth month and Đại Thử its middle qi; the hottest stretch is split into minor and major, the start of the month being minor and the middle being major[3].
It quotes the Chu thư Thời huấn on the three pentads: "小暑之日溫風至，後五日蟋蟀居壁，後五日鷹乃學習", meaning that on the day of Tiểu Thử warm winds arrive, five days later the crickets shelter in the walls, and five days after that the young hawks learn to fly[3].
In its entry for the middle month of summer (仲夏月), the book records the Nguyệt lệnh line "小暑至，螳螂生", meaning that mild heat arrives and the mantises hatch[3]. Here 小暑 refers to the mild heat of the fifth month, not to the Tiểu Thử term.
Comparing Tiểu Thử with neighbouring solar terms
| Solar term | Type | Solar longitude | Usual start date | Main feature |
|---|---|---|---|---|
| Mang Chủng | Term, opens the Ngọ month | 75 degrees | June 5 or 6 (some years the 4th or 7th) | Name refers to awned grain such as rice and wheat |
| Hạ Chí | Middle qi of the Ngọ month | 90 degrees | June 21 or 22 (some years the 20th) | Longest day of the year in the Northern Hemisphere |
| Tiểu Thử | Term, opens the Mùi month | 105 degrees | July 7 (some years the 6th or 8th) | Heat is clear but not at its peak |
| Đại Thử | Middle qi of the Mùi month | 120 degrees | July 22 or 23 (some years the 24th) | Peak heat, by its name |
| Lập Thu | Term, opens the Thân month | 135 degrees | August 7 or 8 (some years the 6th or 9th) | Start of autumn in the solar-term calendar |
Tiểu Thử in Eastern divination schools
In Tử Bình and Bát Tự
In Tử Bình, the month pillar of a Bát Tự chart changes at the 12 terms, not at the middle qi or on the first day of a lunar month. Tiểu Thử opens the Mùi month: a person born from the start of Mang Chủng up to the start of Tiểu Thử has a Ngọ month pillar, and one born from the start of Tiểu Thử up to the start of Lập Thu has a Mùi month pillar. In 2026 Tiểu Thử begins at 8:49 on July 7, the 23rd day of the fifth lunar month, so a person born after that moment has an Ất Mùi month pillar even though the lunar date is still in the fifth month. In 2025 the term began on the 13th day of the sixth lunar month, so a person born from the 1st of that month up to the start of the term still has a Nhâm Ngọ month pillar. The book Kỳ Môn Chỉ Quy, discussing the solar terms, also cites the practice of fate reckoning: a person born after Lập Hạ has begun within the third lunar month "必作四月算命", must be reckoned as born in the fourth month[4].
In Kỳ Môn Độn Giáp
The book Thuật số Lạc Thư divides the 24 terms into two halves, from Đông Chí to Mang Chủng and from Hạ Chí to Đại Tuyết, and places every three terms in one Later Heaven trigram: Hạ Chí, Tiểu Thử and Đại Thử belong to the Ly trigram in palace 9; each trigram has three lines, each line corresponding to one term[2]. The book Kỳ Môn Chỉ Quy uses the solar terms as the points where the chart changes: each fifteen-day term is split into upper, middle and lower periods of five days each; "如节气未到甲子、己卯、甲午、己酉符头先到，谓之超神", meaning that when the term has not yet arrived but a heading day Giáp Tý, Kỷ Mão, Giáp Ngọ or Kỷ Dậu has arrived first, it is called siêu thần (overtaking)[4].
In feng shui
The book Sử dụng la bàn by Thanh Thủy (2006), in the chapter explaining the rings of the Tam Hợp compass, has a table matching the 24 mountains with the 24 solar terms: on the Thái dương đáo sơn plate, at Tiểu Thử the Sun reaches the Đinh mountain, and the book uses these plates when choosing a date for burial or for erecting a stele[5].
What influence does Tiểu Thử have on health?
In folk belief, Tiểu Thử is when the weather turns hot and humid and heatstroke and stomach upsets come easily, so people remind one another to eat light meals, drink enough water and stay out of the midday sun. This is summer living experience, not medical guidance.
