It's been pretty obvious since the first preview was released in June that the OS would be released at the end of October because:
http://9to5mac.com/2013/09/06/apples-os-x-mavericks-release-planned-for-end-of-october/
For the launch of both OS X Lion and OS X Mountain Lion, Apple released the new operating systems the day following an earning results release. Apple traditionally shares its Q4 earnings numbers during the second half of October, so perhaps Apple will release Mavericks the day following its Q4 results announcement
http://9to5mac.com/2013/09/06/apples-os-x-mavericks-release-planned-for-end-of-october/