Mac owners who have installed the latest public beta, which was released earlier today, may have the final version already on their machines.
The public beta -- sixth in a series that started in late July -- was identified as build 14A388b, which matches the number assigned to the third "golden master," or GM, released Oct. 9 to registered developers.
Apple traditionally finalizes its OS X code several days before official release. Last year, for example, Apple generated the last GM two days before launching OS X Mavericks on Oct. 22.
Apple will host a news conference Oct. 16, when it's expected to introduce new iPads and announce the same-day availability of Yosemite. The company will live-stream the event.
Because Apple typically uses the last GM as the release build, and with only three days to Thursday's event, developers who have upgraded to GM 3 probably have the final on their Macs and so won't need to re-download and re-install it later this week.
The same will likely be the case for those who have applied today's public beta.
Nothing is certain, however, since Apple is on unfamiliar ground: Yosemite was the first operating system that the company previewed to the general public since 2000.
Yosemite's public beta weighs in at 5.2GB, slightly less than Mavericks' 5.3GB, but still a massive download for those on slow broadband connections or whose ISPs meter their usage. The beta download process can be started from Apple's website.
It's unknown whether older editions, including 2012's Mountain Lion, 2011's Lion and 2009's Snow Leopard, can be upgraded directly to Yosemite. Those editions can be upgraded to OS X Mavericks, however, hinting that Yosemite will also support the direct-migration paths.
OS X Yosemite will be a free upgrade from Mavericks, and available from the Mac App Store.