Reading these comments just confirms what I've been sure about for years - that there's total confusion about this. It's beyond me how the English speaking world got to the moon when not only can we not agree about what "next Wednesday" means but when most people don't even realise what a minefield for miscommunication it is!
I avoid using the word "next" in this context as much as possible, try to use language that can't be misunderstood and - to their bemusement - almost always check what exactly people mean.