TL;DR: Nguyệt Kỵ is the three days of the 5th, 14th and 23rd of each lunar month that folk custom treats as unsuitable for major undertakings, while Dương Công kỵ nhật is thirteen days scattered through the year that old date-selection books list as days to avoid travel and major undertakings. Hiệp Kỷ Biện Phương Thư, the Qing dynasty date-selection manual, places Dương Công kỵ nhật in its Biện ngoa (refuting errors) section to reject it, while keeping Nguyệt Kỵ in its section on principles with the explanation that it avoids the number 5 of the central palace. Reading these two entries closely shows that earlier scholars drew a clear line between old rules with a basis and folk theories invented by fortune-tellers for profit.
What are Nguyệt Kỵ and Dương Công kỵ nhật?
Nguyệt Kỵ (月忌) and Dương Công kỵ nhật (楊公忌) are two familiar groups of taboo days in the Vietnamese date-selection calendar, usually printed at the bottom of bloc calendars or in date-selection books sold in bookshops. Nguyệt Kỵ is the 5th, 14th and 23rd days of each lunar month; Dương Công kỵ nhật is thirteen days scattered through the year, starting from the 13th of the first month, then the 11th of the second month, through to the 19th of the twelfth month.
What few notice is that both groups are discussed in Hiệp Kỷ Biện Phương Thư (欽定協紀辨方書), the great date-selection manual compiled by Doãn Lộc (Yin Lu), a Qing prince, by imperial order with many collaborators, completed in 1741 and included in the Complete Library of the Four Treasuries[1]. But the way the book handles the two groups is very different, and that is precisely what is worth reading.
What is Hiệp Kỷ Biện Phương Thư and what makes it special?
Hiệp Kỷ Biện Phương Thư is a date-selection manual compiled by the Qing court, in 36 volumes, produced by Doãn Lộc and collaborators and completed in 1741, now preserved in the Complete Library of the Four Treasuries[1].
What makes this book special is that it does not merely list auspicious and inauspicious days like folk almanacs, but has an entire volume titled Biện ngoa (辨譌), meaning distinguishing errors, to refute theories the compilers considered fabricated. In the opening of that volume, the compilers write that fortune-tellers love strange tales and profit, so fabrications flourish; what one calls good another calls bad, and this condition already existed in the Han dynasty, let alone after six dynasties through Tang, Song, Yuan and Ming, when erroneous theories became countless[1].
That passage shows the ancients themselves were aware that the date-selection calendar was a mixed system, built by many schools, full of contradictions. The Vietnamese translation edited by Mai Cốc Thành, translated by Vũ Hoàng and Lân Bình, published by Mũi Cà Mau Publishing House in 1998, also opens with the observation that the art of date selection is a complex system, one might say mixed, because it is composed from many sources, many authors, many schools, and so contains many contradictions[2].
How does Hiệp Kỷ Biện Phương Thư record Nguyệt Kỵ?
The book discusses Nguyệt Kỵ in volume 6, among its Nghĩa lệ (principles and precedents), not in the Biện ngoa volume, which means it keeps the rule as one with a rationale[3].
That entry gathers several older sources. The Lịch lệ says that on Nguyệt Kỵ days the almanac marks only sacrifices, banquets, bathing, grooming, haircuts, trimming nails, seeking medical treatment, mending walls, sweeping the house, repairing roads and pulling down old buildings, and nothing else. The Tề Đông dã ngữ records that custom takes the 5th, 14th and 23rd of each month as Nguyệt Kỵ and avoids everything on them, a view without classical basis; it then quotes Vệ Đạo Phu, who said these three days correspond to the number 5 in the central palace of the Hà Đồ, and 5 is the image of the ruler, so commoners may not use them, which it found fairly reasonable. The Thông thư says that on those days Ngũ Hoàng enters the central palace, paired with the star Liêm Trinh. The Khảo nguyên adds that from the 1st to the 5th is five numbers, and from the 5th to the 14th and from the 14th to the 23rd are each nine days apart, making the numbers nine and five, so people dare not use them[3].
The compilers' note settles it: the meaning of Nguyệt Kỵ lies in the central palace and Ngũ Hoàng, which is right, while the idea of Liêm Trinh generating earth cannot be trusted. Avoiding these days is avoiding what is exalted, just as palaces and government offices use a due north-south orientation that ordinary homes dare not use; for the lowly to offend the exalted is unlucky. The state itself does not use these days either, because the affairs of the state inevitably involve its officials and people[3]. Nguyệt Kỵ is thus kept for a reason of ritual, not because of an astronomical calculation.
Which days are Dương Công kỵ nhật and why does Hiệp Kỷ Biện Phương Thư reject them?
The book places Dương Công kỵ nhật in the Biện ngoa section, the section refuting errors, not in the list of taboo days to observe.
This is the most important point that many bloc-calendar readers never know. Hiệp Kỷ Biện Phương Thư records clearly that most people fear Dương Công kỵ days, almanacs also record them widely, saying such days are unsuitable for travel or major undertakings, and that violating them brings harm, but all this is because people do not understand the origin and so are misled[1].
The compilers explain that origin as follows: according to the transmitted theory, Dương Công kỵ is the day of the star Thất (室) of the Huyền Vũ constellation, that is Thất Hỏa Trư. The method is to start from the star Giác on the 1st of the first month, then count in order through the twenty-eight mansions; the day meeting Thất is called Dương Công kỵ. Regardless of whether a month is long or short, the cycle repeats every twenty-eight days, so each month moves back two days, giving the 13th of the first month, the 11th of the second month, through to the 1st and 29th of the seventh month, and ending on the 19th of the twelfth month, thirteen days in the year[1].
But Hiệp Kỷ Biện Phương Thư immediately refutes this very method. The book argues that using the duty star of the day to judge fortune is like using stems and branches to record days, which, though perhaps not starting from Đại Náo (Da Nao), has existed for long, one star per day, in natural order, without need of arrangement, and using that to discuss avoidance might still be arguable. But no one has ever heard that a day when Thất stands on duty has any taboo. Moreover, the mansion used here is not the one actually on duty that day; it is forced by starting from Giác on New Year's Day, so each year restarts and cannot rotate, which is already incoherent. Further, if the first month is a short month, then the 11th of the second month is the star Nguy, and if the second month is short again, then the 9th of the third month is the star Hư; how can one insist on calling it Thất Hỏa Trư?[1]
The book also cites classics to refute. The Book of Jin says Doanh Thất is the Nguyên palace, also called Thanh miếu, surely not an inauspicious star. The Book of Songs has the line "Định chi phương trung, tác vu Sở cung", speaking of observing the star Định to begin construction, not of taboo. The Erya says Doanh Thất is called Định; the ancients observed Doanh Thất in mid-sky to begin earthworks, never treating it as taboo[1].
And the book concludes with a heavy sentence: if the excuse is that the star belongs to Fire, then Vĩ, Thất, Chủy and Dực all belong to Fire, why taboo only Thất? If the excuse is that the star has the shape of a pig, then stars are images of the celestial vault, not animal shapes; how can one assign it to be actually a pig? That deceitful theory misleads the people more than anything[1].
Why would a date-selection book reject its own taboo days?
The answer lies in the purpose of Hiệp Kỷ Biện Phương Thư. This is a book compiled by the Qing court by imperial order, to unify date selection throughout the realm. Such a book cannot allow hundreds of contradictory taboo theories to coexist, for then each fortune-teller would have his own method, each locality its own rule, and no one could verify anything.
The Biện ngoa volume refutes more than twenty theories, such as the greatly auspicious month for marriage, the twenty-four mountain-patrolling spirits, knife, needle, fire and blood days, red sand, the "ice melts, tiles break, family destroyed" great calamity, Dương Công kỵ, the heavenly dog and the nine-immortals hours[1].
This list shows that Dương Công kỵ nhật is not an isolated case, but one of a series of theories the book places in the category to be refuted. This approach is also a tradition among Chinese researchers of the numerological arts. Tưởng Đại Hồng of the Ming-Qing transition, in his book Bình sa ngọc xích biện nguỵ (平砂玉尺辨偽), refuted the Bình sa ngọc xích kinh as a forged text, using criteria of whether it agreed with the secret essentials of predecessors and whether it proved accurate for famous tombs and houses, to distinguish true from false[4]. The reasoning in Hiệp Kỷ Biện Phương Thư follows the same spirit: argue by principle, compare against the classics, not repeat hearsay.
Comparison table of Nguyệt Kỵ and Dương Công kỵ nhật
| Criterion | Nguyệt Kỵ | Dương Công kỵ nhật |
|---|---|---|
| Number of days | 3 days a month (5th, 14th, 23rd) | 13 days scattered through the year, two in the seventh month |
| Method of determination | The three days match the number 5 of the central palace (Ngũ Hoàng enters the centre) | The day reached at Thất when counting the twenty-eight mansions from Giác on the 1st of the first month |
| Attitude of Hiệp Kỷ Biện Phương Thư | Kept in volume 6 (Nghĩa lệ), explained as avoiding the exalted | Placed in the Biện ngoa section, clearly rejected |
| Reason for rejection | Not rejected | Method cannot rotate, star assignment incorrect, classics show no taboo |
| Prevalence in Vietnamese folk custom | Very common, printed on bloc calendars | Common, usually recorded in date-selection books |
Are Nguyệt Kỵ and Dương Công kỵ nhật Vietnamese knowledge?
Both concepts come from Chinese-language date-selection books: Hiệp Kỷ Biện Phương Thư quotes the Song-era Tề Đông dã ngữ on the custom of avoiding the 5th, 14th and 23rd, and says the almanacs of its time widely recorded Dương Công kỵ[1][3].
In Vietnam, Nguyệt Kỵ and Dương Công kỵ nhật are widely spread through bloc calendars, date-selection books sold in bookshops, and oral transmission. But one must distinguish clearly: these are concepts originating from Chinese date-selection books, introduced to Vietnam through cultural exchange, not indigenous knowledge. This does not diminish their value as part of contemporary Vietnamese culture, but it helps readers understand the context of their formation.
Some popular materials claim Dương Công kỵ nhật is tied to a historical figure named Dương Công, but Hiệp Kỷ Biện Phương Thư says plainly that it does not know who Dương Công was, and suggests the theory may share an origin with the secret-day theory from the Western Regions, where that people disliked pigs, found even the sound repulsive, and so tabooed them, but this cannot be applied to China[1]. This is an example of the ancients tracing the origin of a taboo and trying to explain it through specific cultural context, rather than sanctifying it.
How did the ancients distinguish old rules from folk theories?
The book distinguishes between conventions with a basis in calendrical calculation and theories invented by fortune-tellers for profit, calling the latter false theories and placing them in the Biện ngoa section[1].
The criterion is this: a rule is kept if it has a rationale in calculation or in ritual; a theory is rejected if it rests on arbitrary assignment, cannot rotate, or contradicts the classics. Dương Công kỵ nhật is rejected because its method cannot rotate and its star assignment is incorrect. Nguyệt Kỵ is kept because the book finds its meaning in the central palace and Ngũ Hoàng, though the convention itself remains a convention of date selection, not a natural fact.
This distinction is also a lesson for readers of date-selection calendars today. When a calendar marks a day as taboo, the reader should ask: how is that day determined, does it have a basis in calendrical calculation, or is it merely hearsay? And if it is hearsay, which group does it belong to: an old rule with a basis, or a folk theory invented by fortune-tellers?
Nhật Nguyệt's suggestion
When consulting a calendar to choose a day for a major undertaking, readers may refer to Nguyệt Kỵ and Dương Công kỵ nhật as part of old rules, but should know that Hiệp Kỷ Biện Phương Thư itself, the date-selection book authorized by the Qing court, placed Dương Công kỵ nhật among false theories to be rejected. This does not mean the two concepts must be abandoned, but that their position should be understood correctly: Nguyệt Kỵ is an old rule that the book itself kept and explained, Dương Công kỵ nhật is a folk theory that the old date-selection book itself doubted. For important matters such as marriage, groundbreaking, or signing contracts, the decisive factors remain practical preparation and agreement among parties, not a day marked taboo on a calendar.
