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The days basically follow the cycle from Sensho to Shakku, as listed on our About Rokuyo page.
There are exceptions; the first days of months in the 4,000-year-old Chinese lunar calendar are always assigned the same Rokuyo day:
| Lunar month | First day | |
| 1 and 7 | 先勝 – Sensho | |
| 2 and 8 | 友引 – Tomobiki | |
| 3 and 9 | 先負 – Sakimake | |
| 4 and 10 | 仏滅 – Butsumetsu | |
| 5 and 11 | 大安 – Taian | |
| 6 and 12 | 赤口 – Shakku |
The second day is the same as the next Rokuyo month's first day.
For example, in month 9, the 1st day is Sakimake, the 2nd day is Butsumetsu, and so on to the 6th day of Tomobiki. Then the cycle starts again at the 7th day of Sakimake.
See the pattern? (If not, look at the calendar displaying the lunar days.)
That's basically it, though there are a few banana skins, as we'll now explain.
The six-day cycle is not regular. As you see in the calendar page the cycle is reset at the start of each lunar month. Why? Because that's what it does. You might remember from junior school that the average length of a synodic month is 29.530589 days. And that means two things:
The first day of a lunar month is the day when an astronomical new moon occurs in a particular time zone. Which time zone? Well, assuming you're interested in the Rokuyo days for your time zone, then it's your time zone for the new moon which matters. No need for you to worry about that.
However, be aware that a lunar "month" is not a whole number of earth days, ranging 29 days 8 hours to 29 days 19 hours. As mentioned above, this is rounded up or down to 29 or 30 days, corresponding to the period of lunation. For regions at different longitudes (China and Japan, for example) and as different cultures used different astronomical data, a different day could be calculated as the start of the lunar month.
Complicating matters further, a leap month is added every few years to keep the calendar aligned with the seasons and solstices. Fortunately, there are innumerable lunar calendars on the internet you can copy and paste from.
For today's calculations of Rokuyo, it's a simple case of first superimposing your lunar calendar over the current Gregorian calendar (with its different number of days each month: 28, 29, 30 or 31), and then reaching for an aspirin to soothe our nerves.
Focus too much on the 'time' thing and we risk drifting into the esoteric nature of this indefinite and irreversible succession of things and events from the past through the present to the future, start chanting weird things and get taken away by burly staff in white coats.
But if you've survived so far with the above, then the remainder is a doddle.
Rokuyo is simply assigned by simple arithmetic of four numbers:
(lunar month + day) ÷ 6 = result
For example, the Rokuyo day for 19th day of the tenth month of the lunar calendar is calculated using (19 + 10) ÷ 6. That's 29 ÷ 6 = 4, with 5 remainder.
Then we look at the table to see that a reminder of 5 is Butsumetsu.
| Remainder = 0 | Taian |
| Remainder = 1 | Shakku |
| Remainder = 2 | Sensho |
| Remainder = 3 | Tomobiki |
| Remainder = 4 | Sakimake |
| Remainder = 5 | Butsumetsu |
Another example, the Rokuyo day for 4th day of the third month is (4 + 3) ÷ 6. That's 7 ÷ 6 = 1, with 1 remainder and the table shows that a reminder of 1 is Shakku.
In other words, the Rokuyo day is simply determined by remainder number.
Fortunately there's no need for you to calculate this for yourself at all, because we publish the Rokuyo for every day of the years 1900 to 2100 on our 200-year calendar: rokuyo.org/rokuyo/calendar-lunar.php, and the simpler calendar without the lunar days at rokuyo.org/rokuyo/calendar.php).