Pond Boss Forums AQUATIC VEGETATION Controlling unwanted plants Need to make Duckweed disappear for short time

If you aren't too concerned about anything other than clearing it out, just hit it with a copper based product like Cutrine, probably from any local farm store. Dredging the lake and aerating sound like good steps to eventually getting it cleaned up, too. But for now, hit it with a chemical killer immediately and you'll have plenty of time to monitor and manage your results before the celebration week.

You'll probably get some other idea here as well, but that'd be my vote for a quick solution that will probably last more than a week.