This post is a result of a tweet  by Space Rogue.
Has anyone published any etymology on the origins of the term '0-day'? Google not helping. #getoffmylawn
— Space Rogue (@spacerog) October 8, 2013
It seems that only a few  have tried to capture the origin of the word 0-day in hacking and are wrong.
The term 0-day comes originally from the Warez scene :
“0-day (pronounced as zero day) – This refers to any copyrighted work that has been released the same day as the original product, or sometimes even before. It is considered a mark of skill among warez distro groups to crack and distribute a program on the same day of its commercial release.”
Somewhere around the late 90’s it was picked up by the hacking scene.
Wikipedia explains zero-day attacks as following :
A zero-day (or zero-hour or day zero) attack or threat is an attack that exploits a previously unknown vulnerability in a computer application, meaning that the attack occurs on “day zero” of awareness of the vulnerability. This means that the developers have had zero days to address and patch the vulnerability. Zero-day exploits (the software and/or strategies that use a security hole to carry out a successful attack) are used or shared by attackers before the developer of the target software knows about the vulnerability.
The earliest recorded mentioning of 0-day, related to the hacking scene, so far is an e-zine of 1998 called CRH , (thanks to @bill_e_ghote for finding this one).
Other references to 0-day in 1998 include a post on BugTraq by Ken Williams  and the Line-noise section of Phrack 53 .
Let me know if you have found a reference to 0-day (in hacking) before January 31st, 1998. A good place to start looking might be newsgroups, but I have been unlucky so far.
Thanks go to
Space Rogue, twitter: @spacerog website: http://www.spacerogue.net
Bill E. Ghote, Twitter: @bill_e_ghote website: http://scrapeghote.blogspot.com
 https://twitter.com/spacerog/statuses/387677286385733632 by @spacerog on October 8, 2013.
 http://spiresecurity.com/?p=576 – “Zero Day” Terminology by Pete Lindstrom on July 27, 2005.
 http://web.textfiles.com/ezines/CRH/crh007.txt – 7th edition of CRH E-zine published on January 31st, 1998
 http://www.shmoo.com/mail/bugtraq/oct98/msg00027.html – Bugtraq October 5th, 1998
 http://www.textfiles.com/magazines/PHRACK/PHRACK53 – Phrack 53 July 8th, 1998