If these labels are written according to the usual spelling of some actual language/dialect, is there anything we can say about this language? This looks like a Romance language, however many languages seem to accept Roman month names (like in English or German), while not being Romance languages at all, so I suppose we can't really say anything about in which language the person who wrote these month names intended to write them?
Assuming for a moment that the language of the labels is related to the hypothetical plaintext language of the manuscript, do the labels put any restrictions at all on which plaintext languages we should consider?
Assuming for a moment that the language of the labels is related to the hypothetical plaintext language of the manuscript, do the labels put any restrictions at all on which plaintext languages we should consider?