For Halloween, Samhain, whatever you want to call it all Universal Nexus titles are free!
31 Oct – 4 Nov.
This applies only to the ebooks, and only to retailers that allow it.
Amazon allows free books, but only if they elect to make them free (or are exclusively on Amazon and you have limited number of promo days). So the books might go free there — eventually, when Amazon decides to price match the other retailers, and will end when they decide to end it.
Nook doesn’t allow books to be set to free, so it’s out altogether — sorry, take it up with Barnes & Noble.