The desktop localization event focuses on checking translation completeness and quality across the most visible desktop applications. You can help out if you can read any language other than English - just head over to the test day page, check the translations for the listed applications in your language, file bugs if you find any errors, and fill out the results table.
OpenStack is an open source cloud stack which has gained a lot of momentum and support across the industry, and Fedora is working to include OpenStack in Fedora 17. It looks like the Test Day organizers have come up with a set of test cases to work through the entire process of setting up an OpenStack-based cloud and deploying an instance on it, so this will be a great event for ensuring Fedora has a fully working OpenStack...stack...and could even work as an introduction to OpenStack if you've always been interested in it but haven't had the chance to dive in yet.
You can even contribute to both events without ever installing Fedora on real hardware - apparently it's possible to run OpenStack on a virtual machine, which is really just showing off. So you have no excuse for not coming along and helping ensure Fedora 17's desktop translations are in good shape, and that it can act as a great platform for OpenStack deployments!
We'll hope to see you there in #fedora-test-day on Freenode IRC - you can use WebIRC to connect if you're not a regular IRC user. There will be experienced members of the relevant teams on hand to help you out with testing and reporting your results all day long. Let's get the Fedora 17 test day cycle off to a great start!