Month names collection / metastudy

[Koen G]

While looking into this, I came across Français et dialectes chez les auteurs belges du moyen age by JULES FELLER.
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My French reading comprehension is slow and unreliable, so I asked Gemini for a summary:

For Jules Feller, medieval Belgian authors were not trying to write in their local dialects (Wallon or Picard); rather, they were striving to write in French. However, because they did not have a perfect mastery of the central language, they attempted to deduce or "manufacture" French words by applying logical conversion rules or patterns of resemblance.

The creation of the word octembre is a perfect example of this phenomenon: the human mind spots a regularity in a series of neighboring words (Septembre, Novembre, Décembre) and, through a pure logic of assimilation (analogy), alters the word octobre to align it with the others.

Feller thus categorizes octembre as "false French manufactured by analogy" or part of a "hybrid language"—a form that is neither pure dialect nor correct French, but an artificial creation born out of a well-intentioned (though clumsy) effort to write correctly.

The dialects in question are Walloon and Picard. 

This confirms our suspicions that there is some cross-linguistic/dialectical tension on these month names. 

So I wonder if the temporal component ties in to the dialectical: decreased influence of the dialects as the standard language became more widespread?