TL;DR: Cửu Độc is a set of nine days in the fifth lunar month: the 5th, 6th, 7th, 15th, 16th, 17th, 25th, 26th and 27th. The sixth-century Kinh Sở tuế thời ký only calls the fifth month the evil month, with taboos on sunning beds and mats and on roofing a house; the list of nine days appears in the taboo-day table of the Thọ khang bảo giám, a Republican-era book urging moral discipline, where what is avoided is marital relations. In 2026 the nine days fall on 19 to 21 June, 29 June to 1 July and 9 to 11 July. This is a religious belief, not a scientific fact.
What is the Cửu Độc day of the fifth lunar month?
Cửu Độc (九毒日) is a set of nine days in the fifth lunar month: the 5th, 6th, 7th, 15th, 16th, 17th, 25th, 26th and 27th. It is not a calendar system but a taboo tied to the fifth month, regarded as the toxic month of the year; the number nine means nine days, not nine kinds of poison.
Two layers of sources need to be kept apart. The first is the idea that the fifth month is a bad month, already present in the sixth-century Kinh Sở tuế thời ký. The second is the list of nine days: it appears in the "giới kỳ" table (days on which marital relations are to be avoided) of the Thọ khang bảo giám (壽康寶鑑), a book urging moral discipline printed in China in the Republican era, and not in the Kinh Sở tuế thời ký, the Tuế thời quảng ký or the Hiệp kỷ biện phương thư.
Origins in the Kinh Sở tuế thời ký and the customs of the fifth month
The book Kinh Sở tuế thời ký (荊楚歲時記) by Tông Lẫm, sixth century, records seasonal customs of the Kinh Sở region, including the fifth month and the Đoan Ngọ custom. This is an ancient Chinese-language text recording the customs of a region of China, not a description of Vietnamese customs, so it is used only to compare origins and must not be taken as evidence for Vietnamese custom[1].
The book opens its fifth-month entry with: "五月俗稱惡月，多禁。忌曝牀薦席，及忌蓋屋。", meaning the fifth month is popularly called the evil month, with many prohibitions: do not sun beds and mats, and do not roof a house[1]. The book calls the whole month evil and does not single out any day within it as more toxic than another.
Why is the fifth lunar month regarded as the toxic month?
In folk belief, the fifth month is midsummer: hot, rainy, with insects and snakes multiplying, so it is called the toxic month, the evil month. The Kinh Sở tuế thời ký explains the taboo with a story: the commentary by Đỗ Công Chiêm of the Sui dynasty cites the Dị uyển, which tells of Dữu Thực of Tân Dã who sunned his mat in the fifth month, saw a dead child on it that then vanished, after which his own son died, and suggests the taboo "或始於此" (perhaps began from this)[1]. The same commentary cites the Nguyệt lệnh, which says one may go up to high places in midsummer, and calls the taboo on climbing onto roofs "正與禮反" (the very opposite of the rites)[1].
The nine Cửu Độc days specifically and how they are counted
The nine days fall into three clusters of three consecutive days. The cluster names sơ độc, trung độc and mạt độc (初毒, 中毒, 末毒) come from present-day Chinese articles and do not appear in the giới kỳ table of the Thọ khang bảo giám.
| Group | Lunar day | Common name | Note |
|---|---|---|---|
| Start of month | 5th, 6th, 7th | Sơ độc | Coincides with Đoan Ngọ and the two following days |
| Middle of month | 15th, 16th, 17th | Trung độc | Falls in the full moon week |
| End of month | 25th, 26th, 27th | Mạt độc | Near the end of the month, before the 30th or 29th |
The nine days are counted by lunar date, not by can chi, so each year one only needs to convert them to the solar calendar. Solar dates for 2026 to 2030 (the regular fifth month, Vietnam time):
| Year | 5th to 7th | 15th to 17th | 25th to 27th |
|---|---|---|---|
| 2026 | 19/6 to 21/6 | 29/6 to 1/7 | 9/7 to 11/7 |
| 2027 | 9/6 to 11/6 | 19/6 to 21/6 | 29/6 to 1/7 |
| 2028 | 28/5 to 30/5 | 7/6 to 9/6 | 17/6 to 19/6 |
| 2029 | 16/6 to 18/6 | 26/6 to 28/6 | 6/7 to 8/7 |
| 2030 | 5/6 to 7/6 | 15/6 to 17/6 | 25/6 to 27/6 |
2028 also has a leap fifth month (the 5th day of the leap fifth month is 27/6/2028). The giới kỳ table does not say whether a leap month counts, so the table above lists only the regular fifth month.
Where do the schools and books disagree?
The most visible difference concerns the 14th. The giới kỳ table of the Thọ khang bảo giám does not include the 14th among the nine days, but marks the Tý hour of the night of the 14th as the moment when "heaven and earth unite" (天地交泰), on which marital relations must also be avoided; many present-day articles therefore count ten days.
The second difference is what is avoided: the taboos often heard today, such as avoiding weddings, groundbreaking, opening a business or long journeys, are not in that giới kỳ table nor in the Kinh Sở tuế thời ký.
What does folk belief say to avoid on Cửu Độc days?
In the giới kỳ table of the Thọ khang bảo giám, what is avoided on the nine days is marital relations, and beside each day the book warns that breaking it brings early death or sudden misfortune. For the fifth month as a whole, the Kinh Sở tuế thời ký records avoiding sunning beds and mats and roofing a house[1]. These are religious beliefs with no scientific basis.
How is Cửu Độc related to Đoan Ngọ?
Đoan Ngọ is the 5th day of the fifth month, which is also the first Cửu Độc day. Phan Kế Bính, in Việt Nam phong tục (1915), describes the Vietnamese Đoan ngọ festival: early in the morning people eat rượu nếp (fermented glutinous rice), eggs and fruit, and adults drink liquor called "killing the worms"; at noon they offer a meal to the ancestors, then gather "fifth-day leaves" to dry and later brew; some weave mugwort into the zodiac animal of the year and hang it over the door to ward off misfortune. The Kinh Sở tuế thời ký records similar customs in the Kinh Sở region: on the 5th, people "採艾以為人，懸門戶上，以禳毒氣" (make mugwort figures and hang them on the door to dispel poisonous air), race boats, gather medicinal plants, and "以五綵絲繫臂，名曰辟兵，令人不病瘟" (tie five-coloured threads on the arm, called "warding off weapons", so as not to catch epidemics)[1].
Cửu Độc is not a festival of its own; only its first day coincides with Đoan Ngọ.
Nhật Nguyệt's suggestions when looking up Cửu Độc days
Nhật Nguyệt's suggestion: to find the Cửu Độc days of a given year, convert the nine lunar dates to that year's solar calendar as in the table above; in a year with a leap fifth month, state whether you follow the regular fifth month only or also the leap month.
