We're not developers or psychics, so we don't actually know. But "any idea" is very broad, and there is certainly an answer to this. Yes it is google-able, which is what I did, but I HATE it when I google something and the answer is "go google it", so I answer these questions anyway. Shout out to you, the true hero, who googled the question next year or so and found this post, as well as everyone who didn't post a repeat question because they saw this thread on the front page.

This is the page google found for me-

http://wiki.spiralknights.com/Dark_Harvest_Festival

In the past, the event has started between the 17th and the 23rd of October, so presumably it will be between those days. We will likely be looking at the 18th, 19th or 20th, as those are Wednsday, Thursday and Friday respectively, but the 23rd is a Monday and that's a possibility as well. It started on Friday the last 2 years, which would mean the 20th, but has in the past started on Wednesday before that. My money is split between the 20th or 23rd, since in more recent years it's began later than earlier but anything is possible. They could even just decide not to run the event, though I doubt they would.

The current prize box ends on the 31st, so there will likely be no new prize box for the event, though they could run one alongside the pirate one. If they do that it will likely be a rehash of shadow or apocrea or something, which is fine and a nice chance for a handful of people to buy some or what have you. Last year they ran the dangerous prize box ending on the 18th, a few days before the Dark Harvest Festival began. There was no halloween box for sale during last year's dark harvest festival, I do not believe.

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Past predictions-

2015- http://forums.spiralknights.com/en/node/112302
Me (AKA No-More-Starlight) = Wednesday the 22nd
Actual = Thurs. the 23rd

2016-
No thread was made that I could find, but I posted this snarky image- https://imgur.com/fWk1VyU