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

1. 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.

2. 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.

3. 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.

4. Trust as a moat. Competitors copy each other’s text. Sourced content, a verified engine and clear evidence labels are slow to copy.

5. 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 🙏