Hold a wedding on Taian; avoid Tomobiki for a funeral. The rokuyo — the six-day cycle printed on almost every Japanese calendar — look like superstition, but nobody decides them day by day. They are computed mechanically from the month and day of the old lunisolar calendar, using nothing more than an addition and a remainder. Once you see the rule, you can explain why calendars sometimes seem to "skip" a rokuyo.
Six labels, always in this order
The six are Sensho → Tomobiki → Senbu → Butsumetsu → Taian → Shakko. Each lunar-calendar day advances one step, cycling every six days. The first day of the first lunar month always starts on Sensho.
The formula: (lunar month + lunar day) mod 6
A day's rokuyo is fixed by the remainder when you divide (lunar month + lunar day) by 6:
| Remainder | 0 | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|---|
| Rokuyo | Taian | Shakko | Sensho | Tomobiki | Senbu | Butsumetsu |
Lunar 1/1 → 1+1 = 2 → Sensho. Lunar 3/15 → 3+15 = 18, remainder 0 → Taian. The Rokuyo calendar first derives the lunar date by astronomical calculation, then applies this rule.
Why calendars "skip" a rokuyo
If the cycle advanced by one every day, Butsumetsu would always be followed by Taian. Yet real calendars sometimes jump — because the counter resets at each lunar month change. When the month rolls over, the day returns to 1 and the rokuyo restarts from (month + 1) mod 6. A real 2026 example (from our own calendar engine):
- Jan 18 = lunar 11/30 → Butsumetsu (11+30 = 41, remainder 5)
- Jan 19 = lunar 12/1 → Shakko (12+1 = 13, remainder 1)
A plain +1 would give Taian, but the month rollover jumps it to Shakko. That discontinuity is the clearest proof that rokuyo ride on the lunar calendar.
Rokuyo don't come evenly
With six labels you might expect about 61 days each per year — but the monthly resets make the counts uneven and year-dependent. Days we counted directly:
| Year | Taian | Shakko | Sensho | Tomobiki | Senbu | Butsumetsu |
|---|---|---|---|---|---|---|
| 2025 | 60 | 61 | 60 | 62 | 61 | 61 |
| 2026 | 61 | 61 | 61 | 61 | 61 | 60 |
| 2027 | 60 | 61 | 61 | 60 | 62 | 61 |
2025 peaks at 62 Tomobiki days; 2026 bottoms out at 60 Butsumetsu days. Year-to-year differences are real once you count them.
How they're used (and the "custom, not science" caveat)
Taian ("all good") is favored for weddings, car deliveries and moving house; venues may be cheaper on Butsumetsu ("all bad"). Tomobiki ("pulls a friend along") is avoided for funerals, and some crematoria close that day. But rokuyo are a form of calendar divination with no scientific basis — a widely used custom for picking dates, nothing more.
FAQ
Can my calendar disagree with yours on a rokuyo?
Do Sensho/Shakko have lucky hours?
The distributions and dates here were computed with the same engine as the Rokuyo calendar (new moons and solar longitude by astronomical calculation, matching Japan's official ephemeris to within about a minute; as of 2026-07-16).