It is October now, so do you say "this December" or "next December", or maybe both? Of course I am talking about the month after next.
If it's clear you're referring to the future, "this December" should be okay all year round. "Next December", however, might be understood both as "the one that comes next", or "the next one after this one", with the second option seemingly being more common.
If it's not that clear - for example, if you're asked "what's the release date for the new Lara Croft game?" - you would risk being ambiguous with "this", but generally wouldn't with "next", if you're closer to the previous December than to the upcoming one.
In October, I'd generally use "this", though if you actually risk offshooting by a year, it's best to just say "December this/next/last year".