Firefox 2008 Download Day is a SpreadFirefox.com project, which is intending to set a Guinness world record for most software downloads in 24 hours! All you have to do is to pledge on their page that you will download a copy of Firefox 3 on the Download Day (the date to be announced later), and actually do download Firefox 3 on that date! You can leave your email address in order to be notified of the exact schedule of the Download Day.
Of course, you can do more than that! You can get your family, friends, colleagues, etc. to download Firefox on that day, and you can get involved in this project in several ways, one being hosting the logo on your blog/web page (yes, see the top of this very page!). It's way cool to set a world record, so jump in and join us in this project. You'll be proud.