TL;DR: Auspicious and inauspicious days classify a whole day according to the twelve deities, while auspicious hours classify each two-hour block within a day according to that same set of deities. The two layers are independent: a day classified as auspicious still contains inauspicious hours, and vice versa. Both are conventions of date-selection texts such as the Hiệp kỷ biện phương thư, not measured astronomical data.
What are auspicious and inauspicious days, and how do they differ from auspicious hours?
Auspicious and inauspicious days are shorthand for classifying an entire day into a good or bad group according to the twelve deities, while auspicious hours classify each two-hour block within a day according to that same set of twelve deities. The two concepts share one system of deities but run on two different time scales: one on days, one on hours. Therefore a day considered auspicious can still contain inauspicious hours, and an inauspicious day still has auspicious hours.
Both layers belong to the art of date selection, that is, choosing good or bad times for tasks, not astronomical calculation. The Hiệp kỷ biện phương thư (the translation by Vũ Hoàng and Lân Bình names Mai Cốc Thành as chief editor) opens its section on date selection by noting that this is a complex system formed from many sources and many schools, and therefore contains many contradictions. This matters when reading any auspicious-inauspicious table: different books may classify differently, and readers should know which convention the article follows.
What are the twelve deities Thanh Long, Minh Đường, Thiên Hình, and Chu Tước?
The twelve deities are twelve spirits assigned to twelve positions, each corresponding to one branch among the twelve earthly branches. In the common Vietnamese naming, the six in the auspicious group are Thanh Long, Minh Đường, Kim Quỹ, Thiên Đức, Ngọc Đường, and Tư Mệnh; the six in the inauspicious group are Thiên Hình, Chu Tước, Bạch Hổ, Thiên Lao, Nguyên Vũ (also written Huyền Vũ), and Câu Trần. This division is presented in the Tử Vi khảo luận of Hoàng Thường and Hàm Chương[1].
How are the twelve deities arranged by month?
The placement rule recorded in the Khảo nguyên and quoted by the Hiệp kỷ biện phương thư: in Dần and Thân months Thanh Long starts at Tý; in Mão and Dậu months at Dần; in Thìn and Tuất months at Thìn; in Tị and Hợi months at Ngọ; in Tý and Ngọ months at Thân; in Sửu and Mùi months at Tuất; then it runs forward through the twelve branches in the order Thanh Long, Minh Đường, Thiên Hình, Chu Tước, Kim Quỹ, Thiên Đức, Bạch Hổ, Ngọc Đường, Thiên Lao, Nguyên Vũ, Tư Mệnh, Câu Trần. For example, in the month established at Dần, the Tý day is Thanh Long and the Sửu day is Minh Đường[4].
The month here is the solar-term month, the month established by the Dipper, which changes on the day of the solar term rather than on the first of the lunar month. The Kiến Trừ method (the twelve trực) also takes this month as its marker, but it is a different cycle: in the month established at Dần, the Dần day is trực Kiến while the Tý day is Thanh Long. A day therefore carries both a trực name and a deity name, and one cannot replace the other.
The table below compares two views of the same set of twelve deities: their auspicious or inauspicious group, and the meaning commonly assigned to each according to the Tử Vi khảo luận[1].
| Deity | Group | Commonly assigned meaning |
|---|---|---|
| Thanh Long | Auspicious | Considered best for all matters |
| Minh Đường | Auspicious | Meeting superiors, promotion |
| Kim Quỹ | Auspicious | Considered good for marriage |
| Thiên Đức | Auspicious | Smooth in all matters |
| Ngọc Đường | Auspicious | Good for documents and study |
| Tư Mệnh | Auspicious | All matters good |
| Thiên Hình | Inauspicious | Avoid litigation |
| Chu Tước | Inauspicious | Avoid litigation |
| Bạch Hổ | Inauspicious | Avoid all matters |
| Thiên Lao | Inauspicious | All matters unfavorable |
| Nguyên Vũ | Inauspicious | Avoid litigation |
| Câu Trần | Inauspicious | Avoid public and household matters |
Are auspicious hours placed by day rather than by month?
Auspicious hours are placed by the branch of the day, and each day has six auspicious hours and six inauspicious hours, each corresponding to a two-hour block. It is the same cycle of deities with a different marker: the Hiệp kỷ biện phương thư records that, taking the day branch as the marker, a Tý day starts Thanh Long at the Thân hour and the Dậu hour is Minh Đường[4]. The Tử Vi khảo luận gives the same kind of example: on a Tý day the Thanh Long hour is the Thân hour; on a Sửu day the Minh Đường hour is the Hợi hour[1]. This is a fundamental difference from the day layer: days are placed by month, hours by day.
The book Phong thủy thực hành trong xây dựng và kiến trúc nhà ở by Vũ Đức Huynh also mentions calculating auspicious hours by day within the month and says this calculation does not change, listing the good stars Thanh Long, Minh Đường, Ngọc Đường, and Kim Quỹ[2]. This book also notes that for groundbreaking and house-building, besides auspicious hours one must also avoid hours considered highly taboo, such as the hour of the master's death, the hour of receiving death, and the Thiên Lôi hour[2]. Thus even in practice there are many layers of taboos overlapping, not just one single auspicious-inauspicious table.
Why can these two layers not replace each other?
Because they run on two different scales, the result of one layer cannot be derived from the other. Someone who has chosen an auspicious day must still check which hours of that day are auspicious hours if they want to select both. Conversely, if one only needs to choose an hour for a small task within the day, one can simply consult the auspicious-hour table without checking the day layer. This is why date-selection books usually print two separate tables: one for good and bad days by month, one for auspicious hours by day.
Do the schools agree with one another?
No. This is where it should be stated clearly rather than quietly choosing one side. The Hiệp kỷ biện phương thư directly notes that date selection is a mixed system formed from many sources and many schools, and therefore contains many contradictions. Regarding the twelve Kiến Trừ deities specifically, this book also says their names and auspicious or inauspicious meanings differ from one person to another, opinions are not unanimous, and it holds that one should combine yin-yang, the five elements, and the specific meaning of each type of task to judge.
Origin and limits of the auspicious-inauspicious tables
On origin, the Hiệp kỷ biện phương thư cites the Lục Thao (a book attributed to Khương Thái Công) and the Việt tuyệt thư to show that the Kiến Trừ theory, which goes together with the yellow-path tables, is old, and concludes that it arose with other theories in the Warring States period, all of them borrowing the name of the Yellow Emperor. A clear distinction is needed: this is a legend attributing authorship, not a proven event. The book records that legend, but the book's recording does not make the legend a historical fact.
It should also be added that the term "hoàng đạo" in date selection is not identical to "hoàng đạo" in ancient astronomy. The Trương Quả tinh tông, in its discussion of Thái Âm cửu hành, uses the term hoàng đạo to refer to the middle path of the Moon, with nine paths including the hoàng đạo and eight others divided into two paths on each side of east, south, west, and north, and tied to the four seasons[3]. This is a use in star interpretation, different from the use in choosing days and hours. Readers should know which layer they are reading to avoid confusing the two meanings.
Nhật Nguyệt's suggestions for using these two tables
Nhật Nguyệt's suggestion: keep the two tasks separate. When choosing a time for an important task, one may consult both the day layer and the hour layer, but should note which book one follows, since books may classify differently. For small daily tasks, consulting the hour layer alone is enough, and one should not postpone all work because an hour is classified as inauspicious. Remember that this is a traditional cultural convention, not a basis for asserting with certainty the outcome of anyone's work, finances, or health.
