No date has been announced afaik, but it is likely to be Tuesday November 1st, since they tend to build up to Halloween and then end it pretty soon after.
2016: 18 Oct – ? (possible: 1 Nov; probable 8 Nov, see below)
Pattern
Ends during the first week of November. (One exception.)
Scheduled updates occur on Tuesdays (very few exceptions since launch)
That suggests 1 November as the most likely date. However, when GW1 had live updates, ANet rarely ended a holiday-themed festival the day after the official holiday. So I’m hoping I expect that it will end on 8 November.
edit: according to Triddy on Reddit, it won’t end on the 1st. I’ve updated the text above accordingly.
John Smith: “you should kill monsters, because killing monsters is awesome.”
So we have a lot of time yet