TL;DR: No birth month is absolutely good for every child. Bát Tự uses solar terms to determine the birth month because the can-chi month is tied to the Sun's position, while Tử Vi Đẩu Số uses the lunar month because its stars are placed according to the lunar month. Folk beliefs about good or bad birth months are only a reference view, not a scientific conclusion.
Which month is good for having a child? A short answer
No birth month is good for everyone. Bát Tự and Tử Vi both read the birth month within the whole chart, and neither treats any month as absolutely good. Sayings about lucky and unlucky birth months are folk beliefs with no verifiable basis.
The two systems also fix the birth month differently: Bát Tự changes months at solar terms, while Tử Vi uses the lunar month.
How is the birth month determined in Bát Tự?
In Bát Tự, the birth month is not counted from the first day of the lunar month but from the solar terms. Of the 24 solar terms, only 12 open a month; they are called tiết lệnh (Lập Xuân, Kinh Trập, Thanh Minh...), while the 12 middle terms between them (Vũ Thủy, Xuân Phân...) do not change the month. Tam mệnh thông hội by Vạn Dân Anh (Ming dynasty) lists each month with its two solar terms, for example "正月寅【立春雨水】", "二月卯【惊蛰春分】", "十二月丑【小寒大寒】": the first month is the Dần month, covering Lập Xuân and Vũ Thủy; the second month is the Mão month, covering Kinh Trập and Xuân Phân; the twelfth month is the Sửu month, covering Tiểu Hàn and Đại Hàn.
The twelve month-opening terms, their start times in 2026 and their range of solar dates from 1900 to 2100:
| Bát Tự month | Opening solar term | Start in 2026 (Vietnam time) | Solar dates, 1900 to 2100 |
|---|---|---|---|
| Dần | Lập Xuân | 4 Feb, 02:55 | 3 to 5 Feb |
| Mão | Kinh Trập | 5 Mar, 20:49 | 4 to 7 Mar |
| Thìn | Thanh Minh | 5 Apr, 01:28 | 4 to 6 Apr |
| Tỵ | Lập Hạ | 5 May, 18:35 | 4 to 7 May |
| Ngọ | Mang Chủng | 5 Jun, 22:37 | 4 to 7 Jun |
| Mùi | Tiểu Thử | 7 Jul, 08:49 | 6 to 8 Jul |
| Thân | Lập Thu | 7 Aug, 18:39 | 6 to 9 Aug |
| Dậu | Bạch Lộ | 7 Sep, 21:41 | 6 to 9 Sep |
| Tuất | Hàn Lộ | 8 Oct, 13:29 | 7 to 9 Oct |
| Hợi | Lập Đông | 7 Nov, 16:50 | 6 to 8 Nov |
| Tý | Đại Tuyết | 7 Dec, 09:49 | 6 to 8 Dec |
| Sửu | Tiểu Hàn | 5 Jan, 15:18 | 4 to 7 Jan |
For example, in 2026 Lập Xuân falls at 02:55 on 4 February. A person born on 3 February 2026 belongs to the Kỷ Sửu month of the Ất Tỵ year; a person born on 5 February 2026 already belongs to the Canh Dần month of the Bính Ngọ year, even though both days are in the twelfth lunar month (the 16th and 18th of the twelfth month of Ất Tỵ). A term can fall up to about half a month before or after the first day of a lunar month, so the Bát Tự month and the lunar month can differ by about that much.
Why does Bát Tự change months at solar terms?
Bát Tự is built on can-chi. The twelve month branches (Dần, Mão, Thìn, Tỵ, Ngọ, Mùi, Thân, Dậu, Tuất, Hợi, Tý, Sửu) follow the Sun's position: each month spans 30 degrees of celestial longitude, and the Dần month begins when the Sun reaches 315 degrees, which is the Lập Xuân solar term.
In the view of mệnh học (the study of destiny), Bát Tự describes how qi moves through the four seasons, and the seasons follow the Sun rather than the Moon, so the month changes at the solar term. Taking the wrong month means a wrong Month pillar, which in turn skews the reading of the day master's strength, the useful god (dụng thần) and the can-chi relationships.
That is why a Bát Tự chart needs the solar date and the birth time: only by comparing them with the moment the term begins can the month be fixed, especially for a birth on the day a term begins.
Why does Tử Vi Đẩu Số use the lunar month?
Tử Vi Đẩu Số places its stars from the lunar year, month, day and hour of birth. The book Tử Vi khảo luận by Hoàng Thường and Hàm Chương states that the birth year, month, day and hour "are calculated by the lunar calendar". The lunar birth month is used in two main places: together with the birth hour it fixes the Mệnh and Thân palaces (the book counts from the Dần palace as the first month forward to the birth month, then counts by the birth hour from there), and it places the month stars such as Tả Phù, Hữu Bật, Thiên Hình and Thiên Riêu. The Tử Vi star itself is placed by the lunar birth day and the Cục, not by the month.
So the birth month in Tử Vi is the lunar month and does not change at solar terms as in Bát Tự; using the solar-term month to place Tử Vi stars mixes the conventions of the two systems.
Table comparing how the birth month is determined in Bát Tự and Tử Vi
| Criterion | Bát Tự | Tử Vi Đẩu Số |
|---|---|---|
| Basis for determining the month | Solar terms (Sun's position) | Lunar month (lunar cycle) |
| Start of the Dần month | Lập Xuân (3 to 5 Feb) | First day of the first lunar month |
| Influence of the birth month | Determines the month pillar, month can-chi | With the birth hour, fixes the Mệnh and Thân palaces; places the month stars |
| Uses solar terms? | Yes | No |
| Reference tools | Perpetual calendar with solar terms, Bát Tự software | Lunar calendar, Tử Vi software |
Are folk beliefs about good and bad birth months reliable?
In folk practice, some people judge a child's birth month by its month branch, checking whether that branch harmonizes or clashes with the parents' ages through the tam hợp, lục hợp and tứ hành xung relations. That is a folk way of reckoning, not a prediction of personality or destiny.
No study has shown that the birth month, whether reckoned by can-chi or by the lunar calendar, determines a person's personality, health or destiny, or makes a family more harmonious. Genetics, upbringing, nutrition and social opportunity are far better studied factors.
Should you choose the birth month of a child according to Bát Tự?
Choosing a child's birth month according to Bát Tự is something some people are interested in, but it should be understood that Bát Tự has no concept of an absolute "good month". Each chart has its own structure, and a birth month may be good for one person but not for another depending on the overall can-chi, five elements, and useful god. Choosing a birth month based on a single factor is not sufficient grounds.
Nhật Nguyệt's suggestion: If you are interested in Bát Tự, treat it as a way of looking things up to understand more, not as grounds for deciding on having a child. The actual date and time of birth are decided by the health of mother and child, as the doctor advises.
How do I know exactly what my birth month is according to Bát Tự?
Compare your solar birth date and time with the table of month-opening terms above: from the start of Lập Xuân to just before Kinh Trập is the Dần month, from Kinh Trập to just before Thanh Minh is the Mão month, and so on to the Sửu month, which opens with Tiểu Hàn. Lập Xuân is also where the year changes: a birth before Lập Xuân still has the previous year's Year pillar. For a birth on the day a term begins, the birth hour is needed to know whether it came before or after that moment. The Child's birth year by parents' ages tool lists the lunar months of each year together with the Bát Tự month-opening terms, so you can look it up yourself.
