Some have called for Sept. 11 to be a "day of service." Now, service is a good thing, particularly in an age where it is possible to be self-centered a great deal of the time. It's important to reflect that we are members of various communities, and have moral obligations to each of them. "Don't be a jerk" is a pretty good maxim to bear in mind.
However, every year has at least 365 days, and I would argue that this particular day is one better devoted to remembering. Remembering that there are those out there who do not feel the need to be part of our greater community, who choose to not play by the rules of civilized society, who believe the ends justify the means - any means, including mass murder - who violate "don't be a jerk" on an unimaginable scale. While these types of people are not necessarily a large percentage of the population, they are still sufficiently numerous that it is a mistake to let our guard down. "Eternal vigilance is the price of freedom," and while that line may not have been designed with airline-hijacking terrorists in mind, it seems appropriate.
Other countries have had their own encounters with the face of terror, on other days of the year, and I am not so egocentric as to want to impose my own nation's particularly eventful day elsewhere. For me, however, September 11 is a day to remember that evil exists, and that good people must not do nothing.