TL;DR: A traditional day is divided into 12 two-hour blocks corresponding to the 12 Earthly Branches, with 6 blocks considered auspicious and 6 considered inauspicious, depending on the day's Branch.
What are the 12 time periods of the day?
In the traditional Eastern reckoning of time, a 24-hour day is divided into 12 time periods of 2 hours each, named after the 12 Earthly Branches: the Tý hour (23:00-1:00), Sửu (1:00-3:00), Dần (3:00-5:00), Mão (5:00-7:00), Thìn (7:00-9:00), Tỵ (9:00-11:00), Ngọ (11:00-13:00), Mùi (13:00-15:00), Thân (15:00-17:00), Dậu (17:00-19:00), Tuất (19:00-21:00), Hợi (21:00-23:00).
A table listing the 12 time periods of the day according to the Earthly Branches and their corresponding hours.
| Earthly Branch | Time period |
|---|---|
| Tý | 23:00-1:00 |
| Sửu | 1:00-3:00 |
| Dần | 3:00-5:00 |
| Mão | 5:00-7:00 |
| Thìn | 7:00-9:00 |
| Tỵ | 9:00-11:00 |
| Ngọ | 11:00-13:00 |
| Mùi | 13:00-15:00 |
| Thân | 15:00-17:00 |
| Dậu | 17:00-19:00 |
| Tuất | 19:00-21:00 |
| Hợi | 21:00-23:00 |
What are the Auspicious Hours?
The Auspicious Hours (Giờ Hoàng Đạo) are the time periods of the day that folk tradition regards as "good", favourable for beginning undertakings (setting out, opening a business, signing agreements, marriage...), as opposed to the Inauspicious Hours (Giờ Hắc Đạo), periods that call for greater caution. Of the 12 time periods in a day, usually 6 are classed as Auspicious and 6 as Inauspicious, based on the Earthly Branch of that day (each day with a different Branch will have a different set of Auspicious Hours).
How are the Auspicious Hours determined by day group and lookup table?
According to the folk lookup table, the 12 Earthly Branches of the day are divided into 6 symmetrical groups (each group of 2 Branches, for example the Tý-Ngọ group, the Sửu-Mùi group...), and each group corresponds to a fixed set of 6 Auspicious Hours. This is a folk convention handed down through perpetual calendar books, not an astronomical calculation.
Points to note when looking up the Auspicious Hours
Because the table of Auspicious Hours is a folk convention and may vary slightly between different perpetual calendar sources, users should treat it as one reference factor within the whole picture (combining it with the age of the person concerned and the type of task intended) rather than as the sole deciding factor.
Do good hours apply equally to everything?
As with good and bad days, the Auspicious Hours are general in nature for undertakings in general; for important matters such as the hour for sending off troops, the hour for breaking ground, or the hour for receiving the bride, those who examine matters carefully often cross-check further against the specific age of the person involved and the type of task, rather than relying only on the general table of Auspicious Hours.
