Lee on Making Light suggests a form of celebration that I find absolutely wonderful: Reclaiming 9/11 parties. So that people who have birthdays or anniversaries or other happy memories associated with that date can celebrate without other folks standing around disapprovingly muttering about How Can You Dishonor The Dead That Way.
As an added bonus, it might help us get over the weird communal PTSD that we as a nation appear to have been suffering from, helped along by the Miserable Failure Administration's constant picking at the scab. That is, we'd be able to look at the date and say, "That was a terrible tragedy. Let's now figure out ways to prevent anything like it from happening again that don't involve lashing out in fear".