Today is the day that Apple acolytes can download the beta release of OS X Yosemite, unveiled at the Apple World Wide Developers Conference last month. The new OS, release number 10.10, will be deployed in its final form sometime this fall.

Until then, Apple is doing something it has only done once before -- making a public beta available to anyone, not just professional developers. Apple last offered a public beta of the original OS X 10.0 over 14 years ago. Back then, Apple charged beta testers $30 for a physical CD-ROM with the software. The beta release of Yosemite can be downloaded at no charge.

Apple is restricting the beta release (would have been fun if the beta release got a distinct name, like Yosemite Sam) to the first million takers. If you preregistered, go to the Mac App Store and plug in your redemption code. If you haven't preregistered, not to worry, because as of this morning the download is still available.

The beta Yosemite has already been put through its paces by developers who have been tinkering with it since the announcement at WWDC, so what the public is getting is quite polished and less likely to blow up your computer.

But caveats abound. Apple and all reasonable observers strongly caution users not to install the beta on a fully functional Mac alongside OS X 10.9 Mavericks -- and Mavericks must be the OS on that computer; the Yosemite beta will not work with older iterations of OS X.

Don't have Mavericks? Users will have to download and install Mavericks (free download) before proceeding with beta Yosemite. If beta Yosemite is to be installed alongside Mavericks, make sure that all system software and third-party software is fully updated, and use Disk Utility to ensure that the hard drive is healthy enough to host another OS.

For safety's sake, install beta Yosemite on either a hard drive partition, an external drive, or -- the best alternative -- on a wholly separate Mac that will operate solely with the beta OS. The latter option also holds the advantage of a quick and tangle-free exit from the beta program if desired.

Next no-no; the beta program will automatically communicate its feelings with Apple directly -- sending info on bugs, problems, crashes, etc. The beta user will not have to get in the middle of this process. That being said, Apple cautions users not to publicly disseminate news, screenshots, and opinions about Yosemite. Sharing is only allowed among other beta Yosemite users. In other words, the first rule about beta Yosemite is that we don't talk about beta Yosemite.

Also; back up your computer all the time, especially before installation of beta Yosemite; back up everything on an ongoing basis, from whatever OS is present on your computer, including beta Yosemite.

OS X Yosemite introduces a lot of new Continuity features, which means greater linkage between iOS devices and OS X. Since these Continuity features require pairing with Apple's forthcoming iOS 8, they are not included in beta Yosemite.