That's the slogan for the Firefox 3 team's attempt to make a new world record for the most number of downloads a software achieves in a single 24-hour period. The date has not yet been fixed, is expected to be within a month's time.

They've already received close to a million pledges. If you'd like to join the effort, just head over to the Firefox Download Day page and sign up. You'll be sent a mail reminding you to download Firefox 3 on the big day.