> That vulnerability in Internet Explorer was known as a “zero-day” because Microsoft, the targeted software maker, had zero days notice to fix the hole when the initial attacks exploiting the bug were discovered.
> The vulnerability underlying CVE-2013-3897 was found internally at Microsoft and would have been fixed in MS13-080. However, in the last two weeks, attacks against the same vulnerability became public, but since the fix was in the code already, it enabled Microsoft to address the vulnerability, CVE-2013-3897, in record time.
https://community.qualys.com/blogs/laws-of-vulnerabilities/2... (unnecessary text omitted)
in warez it was a folder on the ftp server that would list all the latest releases from that day, to save you from navigating all the /pub/whatever folders over your slow 14.4k connection and so that the distributors would only have to grab from one place.
and somehow in this history the pronunciation changed from "oh day" to "zero day" and was re-appropriated as an infosec term.
edit: just read the definition in OP, it is hilarious.