We don’t know the final release date of the final version of Mac OS X Tiger, but “the likelihood is quite high [that it was March 13, 2007] given Apple’s Mac OS X release history as charted here and no prior Mac OS X update has had a version number higher than 10.x.9. It’s easy to observe that the last update to a current Mac OS X version has been released prior to the release of a new version. Also [we] observe that the gap between the last revision of a current and new operating system has shrunk for the past three Mac OS X releases. If Leopard were to follow the same number of days between Panther and Tiger then Leopard would be released on March 27, 14 days after Tiger 10.4.9. I don’t have any confidence in such an early release date,” Switch To A Mac writes.
STAM analyzes much history and evidence and concludes the best guess release date for The OS That Will Replace Mac OS X Tiger as the World’s Most Advanced Operating System” will be on Friday, May 11, 2007.
Evidence, rationale and much more here.