I thought the term 0-day was meant to indicate a exploit conceived of and executed before even a full day had passed from the release of the software being attacked. It seemed to come from the game/software cracking scene where the sooner you crack it the more credential you get. It seems to be getting used to mean "new" or "previously unknown to anti-virus writers" or something relatively mundane. Certainly nothing to do with a "0th day" anything, unless I missed something in the link.