In Vietnamese families, the dates that matter most move every year. A grandparent s death anniversary, T t, the full moon offering: they all follow the lunar calendar, so they land on a different day of the solar calendar each year. Someone in the family has to remember, and tell everyone else. For families living abroad it is even harder.

Nh t Nguy t is the app we built for that.

What it does

• Lunar reminders that repeat correctly. Set a death anniversary once, and it lands on the right solar date every year, including leap months.

• Family groups. Shared reminders, a plan for each occasion, comments and a photo album per event.

• Everywhere you look. Home-screen widgets on Android and iOS, Wear OS and Apple Watch, Google Calendar sync, and a web version.

• Living abroad? Choose whether the calendar follows Vietnam time or your local time.

• Three languages: Vietnamese, English and Chinese.

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

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.

Most calendar apps solve remind me. Ours solves a harder one: remind all of us, on the right day, and help us do it properly.

In many Vietnamese families, death anniversaries, full moon offerings and the first day of T t follow the lunar calendar. The solar date moves every year, sometimes by weeks. The family is spread across cities and countries. And usually one person, often Mum, keeps it all in her head.

Here is what happens when she adds Grandfather s memorial day to Nh t Nguy t. She does it once.

The moment she saves it
She picks 14th day of the 9th lunar month and her family group. Nh t Nguy t converts it to this year s solar date, and next year s, and every year after, using a calendar engine checked against NASA new-moon data. Nobody does the math again.

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?

1. 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.

2. 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.

3. 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.

4. 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.

A while ago someone asked me how long people spend in Nhat Nguyet each day.

I didn t know. And I realised I didn t want to.

Every app is told the same thing: time on screen is the number that matters. Keep them scrolling. Add a feed. Add a streak. I understand why. But that was never the job we signed up for.

Our job is a family s calendar. Grandma s death anniversary. The full moon offering. The Lunar New Year. Dates that move every year and can t be missed. Nobody wants to spend an hour on that. They want it handled, so they can get back to the people the date is about.

A daily need that never goes away. Hundreds of millions of people still organise family life around the lunar calendar: death anniversaries, the Lunar New Year, the 1st and 15th of every month. These dates move every year, they matter to every generation, and someone in each family has to get them right. That is a recurring, emotional, multi-generational need, and today it lives in group chats and paper calendars.

We own the family, not just the user. Nhat Nguyet is built around family groups. One person adds a date, and the whole family is reminded on every screen, with a shared plan, comments and photos for each occasion. Every group is an invitation to new users, and every anniversary brings the family back, year after year. Growth and retention come from the same loop.

Trust is our moat. This category is crowded with apps that make claims they cannot support. We went the other way:

• A calendar engine cross-checked against NASA and the Paris Observatory from 1900 to 2100.

• Over 1,700 articles in three languages, each claim footnoted to books, archives and institutions, each marked as calculated fact, documented custom, folk belief or one school s rule.

• Open datasets with a DOI on Zenodo, GitHub and Hugging Face, so anyone, including AI systems, can verify and cite us.

Most apps about calendars and traditions give you an answer and ask you to trust it. We built the opposite: over 1,700 articles in three languages, where every claim can be traced back to a book, an archive or an institution.

Here are the three ways in.

1. Browse: follow the map

• 7 fields: calendar and astronomy, customs and festivals, regional and ethnic traditions, the I Ching, Purple Star astrology, feng shui, and research that tests these traditions.

• The knowledge map: over 1,000 connected topics, from the basics to advanced, so you always know what to read next.

• A dictionary for the terms you keep meeting: the sexagenary cycle, the five elements, the 24 solar terms.

• Every article tells you what kind of knowledge it is: a computed calendar fact, a documented custom, a folk belief, or one school s rule, each marked in the text and explained here. Footnotes point to the exact source.

Release date 21 Oct, 2026

In a Vietnamese family, a death anniversary or the Lunar New Year is never one person s job. Someone buys the offerings, someone cooks, someone drives the grandparents, and someone has to remember which prayer to read. Until now, that planning happened in a group chat, while the reminder lived in our app.

Version 1.0.9 brings the whole conversation into the reminder itself.

Grandma s death anniversary falls on the 15th of the 9th lunar month, a different solar date every year. One aunt lives in Saigon, a cousin is in Melbourne, and the person who usually remembers is busy that week.

Nhat Nguyet was built so that nobody in the family misses it.

One reminder, the whole family

Add the date once, in lunar terms. Everyone in the family group gets it at the same time, wherever they are: