The octave has to be the eighth day (counting inclusively so Nov. 8 is the octave of Nov. 1).
After seeing the translation in post 11, I think the only problem is in the original article, which leaves out the “Monday after” part and implies that the writer of the letter observed All Saints’ on Nov. 7. That said, it is somewhat curious that All Saints’ is referred to on Nov. 7, and so the other discussion is worthwhile.
Observing an octave can have two meanings—observing the eighth day or observing eight days. In practice, the two often intersected, with Day one having the biggest splash, day eight having the second biggest splash, and days two through seven having a lesser splash yet. The Octave of Christmas as presently observed is a good example of that.
While all western European countries may have been using the same calendar in the secular sense of the word (though one of the earlier posts brought this into question in an interesting way via ides and calends), I am fairly certain that the variation in liturgical calendar was probably significant. November 1 was certainly All Saints pretty well everywhere in the west, but whether it was observed everywhere with an octave, and the same sort of octave, is not something that I’m particularly confident of—I am a theologian who has taught some liturgical history and so have read a reasonable amount in the field, and I would hesitate to draw or believe conclusions without seeing a fair number of documents from the era or at least copious references to such documents.
All that said, this document would certainly point to observance as an octave in some sort in France.