Ask ten Vietnamese families how they hold a death anniversary and you get ten answers. In the Red River Delta many make the first offering the afternoon before; in the Mekong Delta plenty do it on the day itself. T y families invite a Then master, Khmer families go to the pagoda. All of these are real. Almost none of it is written down in one place. What exists online is mostly the same few pages copied and recopied, each copy a little different, none saying where it came from.
So instead of writing more articles, we built a structure first. Every rite in Nhat Nguyet is recorded the same way: its cycle and lunar date, where it s performed, who is honoured, which ethnic group and cultural region it belongs to, the order of the ceremony, the offerings, the taboos. Each of those fields carries its own source.
The sources come in layers. At the bottom are scholarly books, mostly public domain now: Phan Ke Binh s Vietnamese Customs from 1915, Nguyen Lien Phong s 1909 verse record of southern customs, Dao Duy Anh s 1938 cultural history, Toan Anh s work from the 1960s. On top of those, recent research by Ngo Duc Thinh, Tran Ngoc Them and Dinh Khac Thuan. Then heritage files: UNESCO nominations for the Giong Festival, Hung Kings worship and Then practice, plus ritual records from the Hoi An heritage centre. Press articles only count if they come from a list of 124 vetted sources.
Follow one question through 1,791 articles: the Nhật Nguyệt knowledge map
Most sites in this space give you a pile of articles and a search box. We wanted the opposite: a map you can stand on, see where you are, and see what comes before and after.
So here is one question, followed through the map.
“What year of the zodiac am I?”
Foundations. The answer starts with the stems and branches, the sixty-step cycle that names every year, month, day and hour. That node holds 116 articles, from the basic definition to the twelve traditional watch-hours converted to clock time.
Components. One level up are the twelve zodiac animals, each with its own small branch: personality as folk belief describes it, birth years, naming a child born in that year.
Techniques. Next come the relations between branches: the three harmonies, the six harmonies, the six harms, the clashes. These are the rules people use when they compare two birth years.
Applications. At the top sit the things people actually do with it, like checking whether a child born in 2027 suits the parents’ birth years. Every step on that page is computed by the same engine that powers our tools.
Every article you open has a “Read next” block that follows this tree. You never land on a dead end, and you always know whether you are reading a foundation or an application.
The same tree also holds the skeptical view. Next to “What are the twelve zodiac animals?” sit pieces on the Barnum effect, on whether Dragon years really raise birth rates, and on whether children born in an “auspicious” year do better at school. They are not tucked away in a separate “science” section. They live on the branch they question.
The whole map, as of today:
1,791 articles in 52 topics, hung on 576 knowledge nodes.
Eastern traditions (Purple Star astrology, Four Pillars, the I Ching, feng shui, face and palm reading): 736 articles.
Vietnamese culture (festivals, ancestor worship, 54 ethnic communities, life-cycle rites, crafts): 657 articles.
Western and modern systems (tarot, astrology, numerology, personality types): 398 articles.
Every node exists in English, Vietnamese and Chinese, each with its own address, so a link you share opens in the language you shared it in.
If you want to see how a tradition is built rather than just what it predicts, start at the bottom of the tree:
👉 https://nhatnguyet.org/en/ban-do-tri-thuc
Vietnamese customs were scattered across a thousand web pages, so we built one sourced map of them
Ask ten Vietnamese families how they hold a death anniversary and you get ten answers. In the Red River Delta many make the first offering the afternoon before; in the Mekong Delta plenty do it on the day itself. Tày families invite a Then master, Khmer families go to the pagoda. All of these are real. Almost none of it is written down in one place. What exists online is mostly the same few pages copied and recopied, each copy a little different, none saying where it came from.
So instead of writing more articles, we built a structure first. Every rite in Nhat Nguyet is recorded the same way: its cycle and lunar date, where it’s performed, who is honoured, which ethnic group and cultural region it belongs to, the order of the ceremony, the offerings, the taboos. Each of those fields carries its own source.
The sources come in layers. At the bottom are scholarly books, mostly public domain now: Phan Ke Binh’s Vietnamese Customs from 1915, Nguyen Lien Phong’s 1909 verse record of southern customs, Dao Duy Anh’s 1938 cultural history, Toan Anh’s work from the 1960s. On top of those, recent research by Ngo Duc Thinh, Tran Ngoc Them and Dinh Khac Thuan. Then heritage files: UNESCO nominations for the Giong Festival, Hung Kings worship and Then practice, plus ritual records from the Hoi An heritage centre. Press articles only count if they come from a list of 124 vetted sources.
The prayer texts were the hardest part. We collected them from 717 pages and kept every one word for word, no edits, no translation. Many turned out to be near-copies of each other with a few words changed, so those were merged and only genuine variants kept. Every lunar date was recalculated by our own calendar engine, which is checked against NASA new-moon data. Anything without a source was dropped. Rites that belong only to members of a community are listed but their steps aren’t published.
For geography we used Professor Ngo Duc Thinh’s seven cultural regions and the official list of 54 ethnic groups. Today that adds up to 155 rites, 438 prayer texts on their own pages, 232 kinds of offerings, 176 questions and answers, and pages for 7 regions and 16 ethnic groups. Six datasets from it are open under CC BY 4.0.
It’s still uneven, and we’d rather say so. 354 of the 464 prayer texts come from sources that never say which region they belong to; only 10 are tied to the North. Only 29 of the 155 rites have had a second review. 38 of the 54 ethnic groups don’t have a page yet, mostly because very little has been published about them.
That’s the part we’d most like help with. If your family does something the database doesn’t describe, or describes wrongly, we want to hear it, and every correction gets logged on a public page.
https://nhatnguyet.org/en/phong-tuc-viet
Building a business on the lunar calendar 100 million people still live by
In Vietnam the lunar calendar is not folklore. It’s a working tool. Families use it to:
remember ancestors’ death anniversaries, which fall on a lunar date
pick dates for weddings, moving house or opening a shop
prepare offerings for the 1st and 15th of every lunar month
Today this information is spread across ad-heavy sites, paper almanacs and one relative who “just knows”. Most of it has no sources, and plenty of it is wrong.
What we built
33 tools: lunar conversion, auspicious hours, age matching, Tử Vi and Bát Tự charts, numerology and more. Every result shows how it was calculated.
One calculation engine for web, iOS, Android, Wear OS and Apple Watch. We checked it against NASA, IMCCE (Paris Observatory) and USNO data for 1900 to 2100.
About 1,600 articles in Vietnamese, English and Chinese. Claims are footnoted to primary sources, and every article is labelled by how strong its evidence is: calendar fact, history, folk belief, or one school’s rule.
Family reminders on lunar dates, shared with your family group, with Google Calendar sync.
An AI assistant that answers only from our published articles, with citations.
Open datasets on Hugging Face and GitHub, with a Zenodo DOI.
Why we think it’s a business
Built-in retention. A death anniversary comes back every year. Once a family puts its dates in, the app has a reason to exist for decades, not weeks.
Network effect inside the family. One person sets up the group, and parents, siblings and cousins join. Each new member makes the shared calendar more useful.
Two revenue lines. Family subscription plans (more groups, members and assistant use), plus personalized written readings sold in packs of 1, 3, 9 or 99.
Trust as a moat. Competitors copy each other’s text. Sourced content, a verified engine and clear evidence labels are slow to copy.
Room to grow beyond Vietnam. The overseas Vietnamese community keeps these traditions and often has no one nearby to ask. The English and Chinese versions are also our first step toward other lunar-calendar cultures.
Where we need your eyes
We’re strong on product and content, and still learning distribution. If you’ve grown a consumer app around a cultural or religious habit, I’d love to hear:
What drove your early growth: community, SEO, or something else?
How did you price a family plan versus individual purchases?
Try it at nhatnguyet.org. Brutal feedback welcome 🙏