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Written by Branos Carnutodrûidion/Urādos – Gutuatir of BNG

One of the challenges of reconstructing a Gaulish tradition is deciding what to do with incomplete evidence. Sometimes we have little more than a divine name or a fragmentary inscription. In other cases, however, we have something much more substantial. The Coligny Calendar is one of those rare cases. Discovered at Coligny, France, in 1897, the surviving bronze fragments preserve a five-year Gaulish lunisolar calendar. Its months, days, intercalations, and numerous additional notations give us an extraordinary glimpse into how time was organized in ancient Gaul. Among those notations is one particularly important word:

IVOS

Exactly what ivos meant remains debated, although it is commonly interpreted as identifying a festival or period of observance. More important for our purposes is what we can actually see on the calendar.

IVOS occurs again and again, Sometimes it marks an individual day. More commonly, IVOS markings occur in groups extending across several consecutive days. Many of these runs begin near the end of one month and continue into the beginning of the next. A compilation of the five-year calendar, drawing on the reconstructions of Helen McKay and Garrett Olmsted, makes these repeating concentrations particularly easy to see. Whatever occurred during these periods, the calendar appears to preserve something more complicated than a modern calendar filled with individual one-day holidays.

It preserves festival time.

Festival Time Rather Than Festival Days

This became important when we developed the Îuos cycle used within Bessus Nouiogalation. The repeated IVOS markings may represent extended periods of observance rather than a series of unrelated individual holy days. Some sequences continue across several consecutive days and even cross from one month into another, suggesting that at least some Îuoi may have been multi-day festivals.

For a modern community, however, maintaining numerous extended festival periods throughout the year can be difficult. Rather than attempting to reproduce every IVOS-marked day or the full duration of every possible festival period, we looked at the larger patterns created by the markings and selected principal points of observance within them.

The goal was not to reduce each IVOS marking to a separate holiday, but to preserve the larger rhythm of these festival periods in a form that could realistically become part of a living modern praxis.

We did not simply turn every IVOS on the bronze into a separate BNG holiday. Instead, we looked at the larger patterns created by the markings.

• Where does the IVOS sequence occur?
• What is the Moon doing?
• Where does that lunar month sit within the solar year?
• Are there unusual notations surrounding it?
• And where within that larger festival period could we establish a principal observance that could realistically be maintained as part of a living praxis?

Putting the Calendar Back Under the Sky

In Helen McKay’s reconstruction, the Coligny month begins around the first-quarter Moon rather than the new or full Moon. This means that the numbered days of a Coligny month can be related to recognizable phases of the Moon. Her more recent work also proposes a seasonal placement for the calendar by comparing unusual internal notations with significant astronomical points such as the solstices and equinoxes.

Under this reconstruction, the beginning of Samonios falls one lunar month before the summer solstice. Six lunar months later, Giamonios marks the corresponding opening of the winter half of the year. McKay interprets these positions as corresponding with the seasonal places occupied by Beltane and Samhain in later Insular tradition.

McKay’s seasonal placement also reveals a larger pattern among the IVOS markings. Four prominent festival sequences occur at intervals of approximately three lunar months, creating a roughly quarterly rhythm through the year. Under her reconstruction these fall around the summer opening at Samonios, the Rivros–Anagantios period, the winter opening at Giamonios, and the Equos region. McKay compares these seasonal positions with the later Insular festivals of Beltane, Lughnasad, Samhain, and Imbolc. Those names do not appear on the Coligny Calendar; the comparison is an interpretation of where the Gaulish IVOS periods fall within the year.

For BNG, the important part is not identifying these festivals as Beltane, Lughnasad, Samhain, or Imbolc, but recognizing the underlying pattern: festival periods recur through the year in relationship with both lunar and seasonal time.

This gives us three interacting layers: the Coligny month, the phase of the Moon, the position of the Sun within the year

IVOS occurs within that framework.

Rather than occurring on fixed Gregorian dates, BNG’s Îuoi move with the Moon while remaining anchored to the seasonal cycle of the Sun.

From Many IVOS Days to Principal Observances

This is how the Îuoi of Bessus Nouiogalation developed.
(Those that are marking from the Bronze)

Our current cycle includes:

• Cintusamos — Îuos Nantosueltiâs etic Sucelli
  Two first-quarter Moons before the summer solstice.
• Samolitus — Îuos Taranes
  The first-quarter Moon before the summer solstice.
• Cerdolitus — Îuos Gobanon
  The third-quarter Moon preceding Cintumessus.
• Cintumessus — Îuos Lugous
  Two first-quarter Moons after the summer solstice.
• Cintugiamos — Îuos Ogmi
  Two first-quarter Moons before the winter solstice.
• Giamolitus — Îuos Eponiâs
  The full Moon closest to the winter solstice.
• Adbiuos — Îuos Suleuiânon
  Two first-quarter Moons after the winter solstice.
• Uisonnalitus — Îuos Sironiâs
  Just after the new Moon closest to the spring equinox.

These are not eight ancient festival names found written on the Coligny Calendar. The names, the Dêuoi associated with them, and the ritual practices surrounding them belong to the reconstructed and living tradition of BNG. Their calendrical foundation, however, comes from our work with the IVOS markings and the lunar and seasonal structure preserved by Coligny. In developing these observances, we simplified extended festival periods into principal days that could reasonably be maintained by a modern community or individual.

One portion of Coligny illustrates particularly well why we took this approach.

The Rivros Festival Complex

The month of Rivros contains one of the most remarkable festival sequences on the entire calendar. McKay’s recent analysis identifies a notation containing LUG on Rivros Day 13. This is followed by seven days marked IVOS, producing an eight-day period when the opening day is included. Then the festival stops. For five days there is a hiatus centered upon the period when the Moon disappears in the glare of the Sun. Then IVOS begins again. A second eight-day sequence follows, eventually crossing the boundary between Rivros and Anagantios. McKay notes that this latter boundary-crossing sequence resembles the other major quarterly IVOS periods on the calendar.

In simplified form:

Rivros
LUG notation

↓

IVOS · IVOS · IVOS · IVOS · IVOS · IVOS · IVOS
waning Moon

↓

five-day interval
dark / invisible Moon

↓

IVOS · IVOS · IVOS · IVOS · IVOS · IVOS · IVOS · IVOS
waxing Moon

↓

Anagantios

McKay interprets this larger complex in connection with Lughnasad. Importantly, Lughnasad itself is not written on the Coligny Calendar. That is a comparative interpretation based upon the festival’s position, structure, and the unusual LUG notation. For BNG, however, the sequence demonstrates exactly why simply declaring every IVOS-marked day a modern holiday would be impractical.

This single complex potentially encompasses sixteen festival days separated by a five-day interruption.

Rather than attempting to recreate that entire period as continuous modern observance, our calendar identifies principal lunar points within the larger seasonal structure.

This is the basic method behind our Îuoi.

The BNG Cycle

The relationship can therefore be understood like this:

COLIGNY
Extended IVOS markings
IVOS · IVOS · IVOS · IVOS · IVOS

↓

Lunar and seasonal position identified

↓

BNG
One principal Îuos observance

This does not mean that every BNG observance corresponds neatly to one identical IVOS run. The five-year calendar itself is more complicated than that. IVOS sequences move, additional IVOS markings occur, intercalation affects the pattern, and certain astronomical moments receive unusual treatment. The five-year transcription demonstrates considerable variation alongside the repeating structure.

Our goal was not to force the calendar into eight identical boxes.

It was to create a sustainable cycle that remains rooted in the structure we find there.

Attested, Reconstructed, Living

• ATTESTED
  • The Coligny Calendar exists.
    IVOS occurs repeatedly upon it. It appears on individual days as well as in extended sequences, some of which cross month boundaries. These markings form observable patterns across the five-year calendar.

• RECONSTRUCTED
  • What those periods meant, what occurred during them, precisely how every notation should be interpreted, and how the calendar was positioned within the solar year require reconstruction. Modern scholars have proposed different solutions. McKay’s recent work provides the astronomical and seasonal model currently underlying much of our own understanding, including her placement of the Samonios-opening festival one lunar month before the summer solstice and her interpretation of the complex Rivros festival.

• LIVING TRADITION
  • This is where BNG enters the picture.
    Cintusamos, Samolitus, Cerdolitus, Cintumessus, Cintugiamos, Giamolitus, Adbiuos, and Uisonnalitus are the observances we have developed within this framework. Their names and divine associations are part of our living tradition. We do not claim that an ancient Gaul standing before the Coligny Calendar called one of these days Îuos Taranes or Îuos Suleuiânon.
    We are open about the process. We begin with what survives. We study its patterns. We consider what scholarship can reasonably tell us. Then we build something that can actually be practiced.

Keeping the Rhythm

Reconstruction does not mean that every gap must be filled or that every modern practice must be presented as something inherited unchanged from antiquity. But uncertainty does not require us to remain motionless either.

The people who created the Coligny Calendar left us something extraordinary. Across those fragments of bronze we can still follow months turning into months, lunar phases progressing through their cycles, seasons changing, and certain days being repeatedly marked IVOS. We may never know everything that happened during those days. What we can do is listen carefully to the rhythm they left behind. For Bessus Nouiogalation, the Îuoi are one attempt to do that. We have taken festival periods that could stretch across numerous days and distilled them into principal observances that a modern community can maintain, while keeping them connected to the Moon, the Sun, the seasons, and the surviving structure of the Gaulish calendar.

Sources & Further Reading

McKay, Helen T. “Identifying the Seasonal Placement of the Coligny Calendar.” Études Celtiques 50 (2025), p. 37ff.
McKay’s most recent study of the calendar’s position within the solar year. This is the primary basis for the seasonal placement discussed in this article, including the relationship of the calendar to the solstices, equinoxes, and seasonal festivals.

McKay, Helen T. “The Coligny Calendar as a Metonic Lunar Calendar.” Études Celtiques 42 (2016): 95–121.
An important earlier study establishing McKay’s reconstruction of the lunar structure of Coligny and its relationship to the Metonic cycle.

McKay, Helen T. “Defining the Systematic Patterns for the Triple Marks of the Coligny Calendar.” Études Celtiques 44 (2018): 91–118.
A detailed study of the recurring internal notation patterns found throughout the calendar.

McKay, Helen T. “Where Does the Coligny Calendar Sit in the Year?” Pictish Puzzles and Cryptic Celts, April 27, 2025.
A more accessible explanation of McKay’s 2025 seasonal-placement research, including the calendar’s relationship to the solstices, equinoxes, lunar phases, and festival cycle. Read McKay’s article

McKay, Helen T. “Beltane and its Relationship to the Summer Solstice.” Pictish Puzzles and Cryptic Celts, May 1, 2025.
Discusses Samonios and Giamonios as the openings of the summer and winter halves of the year and McKay’s proposed relationship between Coligny and the later Insular seasonal cycle. Read McKay’s Beltane article

McKay, Helen T. “Lughnasad on the Coligny Calendar.” Pictish Puzzles and Cryptic Celts.
Discusses the Rivros festival complex, its IVOS sequences, lunar structure, and the unusual LUG notation that McKay associates with the later Lughnasad season. McKay herself notes that aspects of this interpretation remain open to further research and debate. Read McKay’s Lughnasad article

Olmsted, Garrett S. The Gaulish Calendar: A Reconstruction from the Bronze Fragments from Coligny, with an Analysis of Its Function as a Highly Accurate Lunar-Solar Predictor, as Well as an Explanation of Its Terminology and Development. Bonn: R. Habelt, 1992.
An extensive reconstruction and analysis of the calendar and its notational systems.

Olmsted, Garrett S. A Definitive Reconstructed Text of the Coligny Calendar. Journal of Indo-European Studies Monograph 39. Institute for the Study of Man, 2001.
Useful particularly for consulting the reconstructed text and distribution of daily notations, including IVOS.