LOS ANGELES STOICS

Intercalation

Intercalation

In addition to the holidays, the Stoic Calendar on the LA Stoics website marks every new moon through the year. This is inspired by the ancient Attic calendar, which used the new moons to mark the start of each month. Every month has a traditional name–usually tied to a god in the Greek pantheon. Sometimes, though, there is an extra thirteenth month which is called “intercalary” on our website. What’s that?

Because twelve lunar months are approximately eleven days shorter than a solar year, using a purely lunar calendar would cause the months to creep backwards over the seasons (like the Islamic calendar). So, in order to prevent that, the ancient Athenians used a lunisolar calendar: they tied the start of their year to the summer solstice, thus forcing the months to relate to the seasons. But as the lunar months shift back, there becomes a gap that keeps expanding between the twelfth lunar month and the summer solstice. The timekeeping process of inserting a week or month to fill that gap is called “intercalation” and the time that is inserted is called the “intercalary period”.

How did the ancient Stoics experience intercalation?

Ancient Athenians (and, thus, ancient Greek Stoics) actually used their calendar to keep track of their many religious festivals and to anticipate days when they were not allowed to conduct normal business or do labor. There were important festivals associated with harvesting crops and sacrificing animals, which the Greeks seemed to prefer to handle earlier in the year rather than later if given the choice (the reasons for this probably have to do with threats of inclement weather in late autumn but I’m not certain). Regardless of the reason, the ancient Greeks did not like the first month, Hekatombaion, starting too far after the solstice and so they adopted a complicated system to avoid this:

  • Normally, Skirophorion would be the last month of the year. If it ended after the summer solstice, then there are no complications. The next month would mark the start of the New Year and Hekatombaion. For example, in 2025, Skirophorion will end on June 24th (4 days after the solstice). The new moon is on the 25th. So New Year’s is on June 26th (with the sighting of the waxing crescent).
  • If Skirophorion ended before the summer solstice, then the ancient Greeks would prefer to start the New Year early. The next new moon (even if it’s before the solstice) would have marked the New Year’s and Hekatombaion. For example: in 2026, Skirophorion will end on June 13th (a week before the solstice on June 21st). The new moon is on the 14th. So New Year’s would be on June 16th according to this system.
  • The following year will see Skirophorion ending –and Hekatombaion beginning– even earlier. After two or three years of pushing New Year’s back, the gap between the end of Skirophorion and the summer solstice would be about 30 days, long enough for a full month. Thus, the Athenians would then insert a full intercalary month, what we may call a “leap month,” since it places the year back in line with the sun. For example: in 2028, the end of Skirophorion will be May 23rd (29 days before the solstice). According to the ancient system, there would then be an intercalary month inserted that year and the New Year’s will would be after the next new moon on June. 24th.
  • What did they call the intercalary “leap month”? Ptolemy reported that the ancient Athenians re-used names: Poseideon or Skirophorion perhaps. After the solstice, the year would start again with the month of Hekatombaion.

This system was used for a time in the 4th century BCE (the century where we see Stoicism founded and thriving in Athens). But, don’t worry, this is not the system that the LA Stoics use.

How do modern Stoics deal with intercalation?

Modern Stoics don’t use the Attic Calendar to keep track of monthly business and they aren’t anxious to ensure the proper dates to harvest crops or give animal sacrifices. That being the case, it’s OK if Hekatombaion sometimes begins in mid-July (as it did in 2023). If Skirophorion ends before the summer solstice, we go ahead and insert an intercalary month that will extend until the first new moon after the solstice. The New Year’s will always be the day of the first waxing crescent after the summer solstice (confer the Islamic/Muslim Calendar if you’re unsure when the crescent appears). For example: in 2026, Skirophorion will end on June 13th. But instead of starting the New Year early, we will have an intercalary month between June 14th and July 14th.

A special occasion

The intercalary month doesn’t happen every year (2024 and 2025 do not have an intercalary period), so perhaps we can use this month to mark a special occasion–to commemorate the regularity of the cosmos and natural law, acknowledge the distinct relationship of the sun, earth, and moon, which allows for our days, months, years, tides, and seasons.

What should we call the intercalary month?

The intercalary month on the LA Stoics calendar is just called “intercalary.” Perhaps there’s another name we can call it–something having to do with timekeeping itself? Something more modern? We will consider this as a community before settling on anything. We have until June 2026, so stay tuned.