Bi-Monthly calendar alignment

A chart of start and end dates for bi-months composed of 61, 60 or 59 days plus leftovers.

They are aligned to either to the first of March, the day after the Gregorian calendar’s leap day, or to the twenty first of December, about the day of the southern solstice.

from	–01-01/ –01-02	–03-01	–04-29	–06-27	–08-25	–10-23	–12-21
to	–02-28/ –02-29	–04-28	–06-26	–08-24	–10-22	–12-20	–12-31/ –01-01

from	–12-31/ –01-01	–03-01	–04-30	–06-29	–08-28	–10-27	–12-26
to	–02-28/ –02-29	–04-29	–06-28	–08-27	–10-26	–12-25	–12-30/ –12-31

from	–12-30/ –12-31	–03-01	–05-01	–07-01	–08-31	–10-31	–12-30/ –12-31
to	–02-28/ –02-29	–04-30	–06-30	–08-30	–10-30	–12-29/ –12-30	–02-28/ –02-29

from	–12-26/ –12-27	–02-24/ –02-25	–04-25	–06-24	–08-23	–10-22	–12-21
to	–02-23/ –02-24	–04-24	–06-23	–08-22	–10-21	–12-20	–12-26

from	–12-21	–02-19/ –02-20	–04-21	–06-21	–08-21	–10-21	–12-21
to	–02-18/ –02-19	–04-20	–06-20	–08-20	–10-20	–12-20	–02-18/ –02-19

Commentary

59 days is the period of 2 lunar months, about 29.5 days each. This makes it easier to keep track of the lunar cycle during a solar calendar year, if you know the dates in the first bi-month. The intercalary covers solstice to Gregorian calendar New Year.

60 days is a highly factored number, enabling a variety of sub-divisions. However, we typically use just a 7 day week. The intercalary follows boxing day when aligned on First of March, and precedes boxing day when aligned on Twenty First of December.

61 days covers the whole year evenly without an intercalary. When aligned with First of March the bi-months roughly correspond with Gregorian calendar months. When align to the 21 December, they start on the twenty first of Gregorian calendar months, except February.

How many days would you prefer ?

• 59 days for easier lunar tracking in a solar calendar.
• 60 days to enable easier subdivision, other than 7 day weeks.
• 61 days to evenly divide the solar year, without an intercalary period.

What alignment would you prefer ?

• First of March for a rough alignment with Gregorian calendar months.
• Twenty First of December,
  without intercalary to start on 21 of Gregorian calendar months,
  or with intercalary to Boxing Day or New Year.
  

My personal preference is to have an evenly divided solar year aligned to physical phenomena, such as the southern solstice, so I’d go for 61 day bi-months aligned to 21st December. I also quite like the idea of an intercalary period over Xmas, which is a holiday period in my culture, but not everyone’s. Though it would be nice to have a 60 day period to subdivide into 5 or 6 day weeks, culturally the seven day week is unlikely to change anytime soon. A better firt for 7 day weeks is 91 day quarters, discussed elsewhere. The average lunar month is about 29 and a half days, so doubling it to make a 59 day bi-month could help with knowing moon phases in a solar year, but lunar calendars and tide tables are a better fit for that purpose.