Apple has since Lion released each release after announcing it at their quarterly earnings call with the exception of Mavericks which was announced during a keynote for the iPad Air and iPad Mini with Retina. The releases are usually the next day except in the case of the keynote, it was available that day. The conference call for quarterly earnings is scheduled for Monday October 20th. But they have released both Lion and Mountain Lion on Wednesdays. I would expect it on Tuesday 21st but it could be Wednesday 22nd.
10.7 Lion. Conference call Tues July 19, 2011. Release Wed July 20, 2011.
10.8 Mountain Lion. Conference call Tues July 24, 2012. Release Wed July 25, 2012.
10.9 Mavericks. Keynote Tues Oct 22, 2013. Immediate release Tues Oct 22, 2013.
10.10 Yosemite. Conference call Monday Oct 20, 2014. Release expected Tues Oct 21, 2014.