The Big Idea
The Lunisolar calendar divides the year into 13 months of 28 days each. That's exactly 4 weeks per month, every month. The extra day or two needed to complete the year are added at the very end.
Because every month has the same structure, dates always fall on the same weekday. The 1st of any month is always Solday. The 15th is always Solday. Your birthday is the same weekday every single year.
Why "Lunisolar"? The name reflects the calendar's dual nature: the 28-day months echo the lunar cycle, while the 365/366-day year follows the solar cycle. It also hints at the month names (all ending in "-luna") and the first weekday (Solday, from "sol" meaning sun).
The Thirteen Months
Month names use Latin number prefixes combined with "-luna" (moon). You can always tell which month it is just from the name - Quintiluna is the 5th month, Deciluna is the 10th, and so on.
| # | Month Name | Abbrev. | Days |
|---|---|---|---|
| 1 | Monoluna | Mon | 28 |
| 2 | Diluna | Dil | 28 |
| 3 | Triluna | Tri | 28 |
| 4 | Quadriluna | Qua | 28 |
| 5 | Quintiluna | Qui | 28 |
| 6 | Sextiluna | Sex | 28 |
| 7 | Septiluna | Sep | 28 |
| 8 | Octoluna | Oct | 28 |
| 9 | Noviluna | Nov | 28 |
| 10 | Deciluna | Dec | 28 |
| 11 | Ondecoluna | Ond | 28 |
| 12 | Dodecoluna | Dod | 28 |
| 13 | Tredecoluna | Tre | 29 or 30 |
The Seven Weekdays
To avoid confusion with Gregorian weekdays, Lunisolar uses names based on the classical planets - the same system that inspired the original weekday names in many languages.
| Weekday | Named For | Symbol | Abbrev. |
|---|---|---|---|
| Solday | The Sun | ☉ | Sol |
| Moonday | The Moon | ☽ | Moo |
| Marsday | Mars | ♂ | Mar |
| Mercuday | Mercury | ☿ | Mer |
| Jupiday | Jupiter | ♃ | Jup |
| Venday | Venus | ♀ | Ven |
| Saturnday | Saturn | ♄ | Sat |
What Every Month Looks Like
This is the calendar for every month from Monoluna through Dodecoluna - they're all identical:
Notice how the 1st, 8th, 15th, and 22nd are always Solday. This is true every month, every year, forever.
The Extra Days
Twelve months of 28 days gives us 336 days. A year has 365 (or 366 in leap years), so we need one or two extra days. These are added to Tredecoluna, the final month:
- Common years: Tredecoluna has 29 days (28 regular + 1 Extraday)
- Leap years: Tredecoluna has 30 days (28 regular + 2 Extradays)
The Extraday(s) at the end of Tredecoluna sit outside the regular weekly cycle. They're not Solday or any other weekday - they're simply "Extraday." Think of them as bonus year-end holidays that don't disrupt the calendar's perfect structure.
Leap years follow the same rule as the Gregorian calendar: years divisible by 4, except centuries unless divisible by 400. So 2024 is a leap year, 2100 is not, but 2000 was.
Writing Dates
Lunisolar dates can be written in several formats:
- Full: Solday, 15 Quintiluna 2026
- Short: Sol 15 Qui 2026 or just 15 Qui 2026
- Numeric: 2026|05|15 (using | instead of - to distinguish from Gregorian dates)
Frequently Asked Questions
How does this line up with the Gregorian calendar?
Each Lunisolar date corresponds to exactly one Gregorian date within the same year. Monoluna 1 is always January 1. The calendars share the same year number and the same total number of days per year. Use the converter to translate between them.
What about the weekly cycle?
Lunisolar has its own independent weekly cycle. Solday doesn't necessarily align with Sunday - they drift relative to each other. If you need to coordinate with people using the Gregorian calendar, you'll need to convert dates.
When does the year start?
Same as Gregorian - Monoluna 1 = January 1. The calendars are synchronized at the start of each year.
Why new names for everything?
To avoid confusion. If we kept "March" or "Tuesday," you'd never know which calendar someone was using. Distinct names make it instantly clear - if someone says "Quintiluna," you know they mean the Lunisolar calendar.
Has anyone tried this before?
Several 13-month calendars have been proposed over the centuries, including the International Fixed Calendar (1902) and the Positivist Calendar (1849). Lunisolar improves on these by using completely new nomenclature and handling the extra days more elegantly.