六曜 Calendar — 大安, 仏滅 and where the cycle jumps
Prints the 六曜 of any date from the arithmetic that produces it — (lunar month + lunar day) mod 6 — with the old-calendar date beside it and the month laid out so you can see where the cycle skips, which is the only trace the Gregorian calendar keeps of the lunar month underneath.
The six labels a Japanese calendar prints under the date — 大安, 仏滅, 先勝 and the rest — are not astrology and not a lottery. They are the remainder of one addition on the old lunisolar calendar, and this page shows the addition. It also shows the month, because the place where the sequence skips a label is the only trace the Gregorian calendar keeps of the lunar month underneath.
The label under the date is arithmetic on the old lunisolar calendar and nothing else — no astronomy of your birth, no reading of you. What the label is supposed to mean about a day is a folk tradition, and it is quoted here rather than recommended.
The label, and the sum it came from
大安
たいあん · taian
旧暦 2026年 10月2日
10 + 2 = 12; 12 mod 6 = 0 → 大安
大安 · taian
- What the characters say
- “Greatly at peace” — 大いに安し. Earlier written 泰安.
- Where the name came from
- The one label that kept its name all the way from the Tang-dynasty 六壬時課, where 大安 was the first of the six hours. It moved from hours to days and from China to Japan without being renamed.
What Japanese calendars print
Calendars print it as good for anything. It is the day a third of Japanese weddings are held on, which is a fact about bookings rather than about outcomes.
Reported, not recommended. See the sources at the foot of the page for how far this sentence can be traced.
The month, and where the cycle breaks
A shaded day is the 1st of a lunar month. The counter restarts there, and because a lunar month is 29 or 30 days — neither a multiple of six — the sequence usually jumps.
The rule, in one line
Add the lunar month number to the lunar day number and take the remainder on division by six. A leap month keeps the number of the month it repeats. That is the whole system: there is nothing else to know and nothing else to get wrong.
- 0大安
- 1赤口
- 2先勝
- 3友引
- 4先負
- 5仏滅
Which is why the 1st of each lunar month is fixed
- 1月・7月先勝
- 2月・8月友引
- 3月・9月先負
- 4月・10月仏滅
- 5月・11月大安
- 6月・12月赤口
And why 旧正月 is 先勝 in every year there has ever been: 1 + 1 = 2.
The six, in the order they run
先勝 → 友引 → 先負 → 仏滅 → 大安 → 赤口 →
The coincidence that made people think it was Buddhist
旧暦 2月15日 is the traditional date of the Buddha’s death, and it falls on 仏滅 every single year. The reason is 2 + 15 = 17 and 17 mod 6 = 5. The other two great observances land elsewhere — 灌仏会 on 4月8日 is 大安 (4 + 8 = 12) and 成道会 on 12月8日 is 先勝 (12 + 8 = 20) — which is what makes the first one a coincidence rather than a design.
Where the six came from
The oldest layer is 六壬時課, attributed to the Tang astronomer 李淳風, which told the fortune of an *hour* rather than a day. Its six were 大安, 留連, 速喜, 赤口, 将吉, 空亡 — the same sequence, four of the names different.
In China a day-fortune, 小六壬, was derived from it; it is described in 沈重華’s 『通読類情書』 of 1771. In Japan the earliest known form is 『頭書長暦』 by 小泉松卓, 1688, with 大安・立連・則吉・赤口・小吉・虚妄 — again the same order. The present six are first attested in 『万暦両面鑑』, 1747.
六輝 and 宿曜, the names sometimes used instead, are Meiji-era coinages made to avoid confusion with the seven-day week, 七曜.
The government tried to abolish it
殊ニ中下段ニ掲ル所ノ如キハ率ネ妄誕無稽ニ属シ人知ノ開達ヲ妨ルモノ少シトセス
“The middle and lower registers in particular are for the most part baseless and absurd, and do not a little to obstruct the advance of knowledge.” — the imperial rescript attached to 明治五年太政官布告第三百三十七号 (the 改暦ノ布告), 9 November 1872, which replaced the lunisolar calendar with the Gregorian. The “middle and lower registers” are where the almanac printed its fortunes.
A second decree that month forbade the fortune notes outright: from 1873 the abridged calendar was not to carry 歳徳, 金神, the good and bad of days, “or any of the rest of that unfounded talk”.
It did not work. Unlicensed almanacs stuffed with the banned notes — オバケ暦, “ghost calendars” — were circulating again by about 1882. 六曜 itself survived by riding along with the 旧暦 in the official abridged calendar, and because every other fortune note had been purged, it came out of the episode more prominent than it went in.
The argument did not end there either. Some Japanese public bodies decline to print 六曜 on calendars they issue, on the ground that it is a superstition with no basis; the 部落解放同盟 has campaigned against it as a practice that feeds discrimination.
Which old calendar this is
Autumn 2033, where nobody can be right
Between 2033-09-23 and 2033-11-22 there is only one lunar month, and the 天保暦 rule needs two: the month holding 秋分 has to be the 8th and the month holding 冬至 the 11th. The rule has no answer, and the national observatory leaves 2033 blank in its own table rather than pick one. This tool follows the older Chinese 時憲暦 rule — first month without a 中気 after the solstice month — which is NAOJ’s 案1 and what the Japanese calendar association recommended in 2015. A publisher who chose differently prints 友引 for 2033-09-23 where this prints 先負, and 赤口 for 2033-12-22 where this prints 大安.
Sources
- The rule itself is arithmetic and is not from anywhere: (lunar month + lunar day) mod 6.
- The lunisolar calendar is this site’s own, computed from the new moons and the 中気; the tests check it against 国立天文台暦計算室’s published 旧暦8月15日 table for 2011–2050 and against published 六曜 calendars for eight months between 1955 and 2034.
- The 2033 arrangement follows 国立天文台暦計算室, 「旧暦2033年問題について」 (2014), 案1 — the 時憲暦 rule — which is also what the 日本カレンダー暦文化振興協会 recommended in 2015.
- The 1872 texts are primary: 明治五年太政官布告第三百三十七号 (改暦ノ布告) and the 太政官布告 of 24 November 1872. Public domain.
- The name histories and the 吉凶 lines are not ours and we have not read the originals. They are attributed in the Japanese reference literature to 佐藤幸治, 『文化としての暦』 (創言社, 1999), pp. 225–227, which traces the current glosses to 『安政雑書万暦大成』 (1854) as reproduced in 『古事類苑』; the line back through 六壬時課 and 小六壬 is set out in 小池淳一 ed., 『新陰陽道叢書』第4巻 (名著出版, 2021). Where one of those books is the only support for a sentence on this page, the sentence is marked as reported rather than asserted.
How to use it
Read the sum, not the label
The panel under the date gives the 旧暦 month and day, their sum, and the remainder on division by six. 大安 is remainder 0, 赤口 1, 先勝 2, 友引 3, 先負 4, 仏滅 5. There is nothing else in the system — no hidden table, no adjustment, nothing that could be got wrong except the lunar date itself.
Look for the jump in the month grid
A shaded day is the 1st of a lunar month, and the counter restarts there. One label skipped means the month that just ended had 30 days; two means it had 29. No skip at all means a leap month has just begun after a 30-day month, which is the one case where the printed calendar gives no sign that anything happened.
Read the label’s history before its fortune
The panel for each label separates three things: what the characters literally say, what the label used to be called, and what Japanese calendars print about it today. The middle one is usually the most interesting. 先負 used to be written 小吉 and counted as a lucky day; the meaning flipped when the spelling did.
Common questions
Is the Japanese 旧暦 the same as the Korean 음력?
On modern dates, yes, and this tool relies on it: both take the day containing the new moon as the 1st, both place the leap month by the 中気, and both are reckoned at UTC+9. The tests check that claim rather than assuming it — against the National Astronomical Observatory of Japan’s own 旧暦8月15日 for every year from 2011 to 2050, and against eight full months of published Japanese 六曜 calendars from 1955 to 2034. They agree everywhere the observatory prints a date. The one place they cannot is autumn 2033, and that has its own answer below.
Why does 2033 carry a warning?
Because the old calendar has no answer there. The 天保暦 rule requires the month containing 秋分 to be the 8th and the month containing 冬至 to be the 11th, and in autumn 2033 there is only one lunar month between them where two are needed. The national observatory leaves 2033 blank in its own tables rather than choose. This tool follows the older Chinese 時憲暦 rule, which is the observatory’s 案1 and what the Japanese calendar industry’s association recommended in 2015 — but a publisher who chose otherwise prints 友引 on 23 September 2033 where this prints 先負.
Is 仏滅 really about the Buddha?
No. The label began as 虚亡 or 空亡 — “no contest”, a draw — and the 仏 character arrived later by substitution. The thing that makes people think otherwise is a coincidence the page shows the working for: 旧暦 2月15日, the traditional date of the Buddha’s death, falls on 仏滅 every year because 2 + 15 = 17 and 17 mod 6 = 5. The other two great Buddhist observances land on 大安 and 先勝.
Should I book my wedding on 大安?
This page does not say, and no page here will. It reports what Japanese calendars print and where that sentence can be traced to, and it prints the 1872 decree in which the Japanese government called this whole class of almanac note baseless and banned it. About a third of Japanese weddings are held on 大安, which is a fact about bookings and about how full the venues are that day, not about marriages.
Feed it from
Next
These accept what this tool produces, so the result can go straight in.
Lunar Calendar Converter
Converts between lunar and solar dates, handling leap months, and lists a lunar birthday for years ahead.
24 Solar Terms
Computes the year’s 24 solar terms to the minute of 절입시각, with the 삼복 dates and which term today falls in.
Nine Star Ki — 本命星 and 月命星, turned at 立春
Works out the two nine-star numbers from a birth date with every step shown: the digit sum, the subtraction from eleven, the 立春 and 節入り instants to the minute, and the 洛書 magic square shifted into a year plate. It prints no personality reading and no lucky direction, and says at length why not.
Maya Calendar — Tzolkʼin, Haabʼ and Long Count
Converts any date to the three Maya counts using the Goodman–Martínez–Thompson correlation, and shows the arithmetic that makes them turn: 260 days from thirteen numbers and twenty names, a 365-day year with no leap day, and a base-20 tally with one base-18 digit.
Zodiac Compatibility (궁합)
Two birth dates give two animal signs and two day stems. Reports the exact relation between them — 삼합, 육합, 충, 원진, 형, 파, 해 — and the five-element relation of the day stems, each with the traditional verdict.
CSV Cleaner
Trims cells, normalises headers, fixes number and date formats and reports rows with the wrong column count.