Download Day is an attempt to set a world record for the most software downloads in 24 hours and will occur on Firefox 3 launch day. Download Day will coincide with the general availability of Firefox 3. The clock will start ticking when Firefox 3 goes out the door and that should be in June.
All you have to do for now is to pledge to get Firefox 3 during Download Day. As soon as it is available, download it, and you'll get a customized certificate from Download Day Headquarters.