Can this be interpreted as a suggestion, like in English "Why don't you go in October?" or "Why not go in October?", or is it only asking why the person has decided not to go in October?
Could be either. It depends on the intonation. :)
I think maybe if it didn't have the 'jullie' it could be the suggestion?
I have the same question.
"Why don't you go in October" sounds better.
"Why do not you go in October? " is not accepted?
I would presume because its not correct English.