Apple signaled that it will ship OS X Yosemite on Oct. 21; it has scheduled its third-quarter earnings call for the day before.

Although Apple has not yet officially revealed the launch date for Yosemite -- which is also identified as OS X 10.10 -- the earnings call on Monday, Oct. 20, is a clue that the upgrade will appear in the Mac App Store the next day.

In both 2011 and 2012, Apple used an earnings call to announce the availability of OS X Lion and Mountain Lion, respectively. In each of those years, the pertinent conference call with Wall Street analysts was for the second quarter, the one that ended in June, because Apple launched the upgrades the following month.

Both times, Peter Oppenheimer, then Apple's CFO, announced the release date during the early minutes of the call. "We are very excited to be launching Mac OS X Lion tomorrow," Oppenheimer said in the July 2011, for instance.

Apple did not repeat that practice last year with OS X Mavericks, as it held its third-quarter call six days after version 10.9 launched.

Another hint that Oct. 21 will be Yosemite's debut came Tuesday, Sept. 30, when Apple released the "golden master" (GM) of OS X 10.10; in the last three years, Apple has issued the GM 19, 16 and 19 days before the launch of the operating system.

Apple's Yosemite includes a host of new features -- notably "Continuity," an umbrella term for handing off tasks begun on an iPhone or iPad to a Mac -- and struts a refreshed visual look reminiscent of last year's iOS 7 with a "flatter," more minimalist feel.

Yosemite will be a free upgrade for all Mac owners with machines capable of running the OS. Those systems include iMacs from the mid-2007 model on; 13-in. MacBooks from late 2008 (aluminum case) and early 2009 (plastic case) forward; MacBook Pro notebooks from mid- and late-2007 and on; MacBook Air laptops from late 2008 and later; Mac Mini desktops from early 2009 and after; and Mac Pro desktops from early 2008 and later.

Apple's third-quarter earnings call will begin at 2:00 p.m. PT (5 p.m. ET) on Oct. 20. Wall Street will be eager to see sales numbers for the quarter, especially of the iPhone, which went on sale the second half of last month. As usual, Apple will provide a live audio feed for the call from its website.