The Christmas season officially starts on the 11th Nov., the last bonfire spectacular is planned for the 10th.
Whilst the Xmas decs will be up way before this I would imagine that the 11th will be the day that any Christmas themed shows, parades start. But I think the Christmas markets start later than that. I'm sure we'll see a lot more info once they release the prices.
I've been keeping an eye on the schedules for the last couple of years because I wanted to make sure I saw the maximum when I went. From what I can gather it's normally straight after the last bonfire do so I would judge it'll be the second week in November. That's what I'm hoping anyway as I would like to see both the Bonfire Spectacular and the start of the Xmas celebrations. Fingers crossed eh?