While there are a lot of things we shouldn't forget, the events of September 11th fifteen years ago ensures that when you say that in September, you are speaking specifically of the terrorist attacks on America. We must NOT forget. But what do we mean?
We can't forget the sacrifices of brave heroes who chose to do their jobs and put the safety and well-being of others above their own. If we aren't willing to do the same in their honor, I'd say we have a pretty good shot at forgetting their sacrifices. The best way to honor those who have fallen is to ensure something like this never happens again.
We can't forget the values that make us who we are and the ideals we hope to live into. A lot can be said about living as if terror has no grasp on us, but this is precisely one of the reasons we can never forget. More than some kind of hyper-vigilance that is impossible to maintain and will not dissuade evil, I believe our best bet is to hold more closely to who we are and what we hope for rather than trying to avoid who we don't want to be or want to avoid.
We can't forget the evil that coaxed several men to give their lives in the pursuit of taking the lives of others. This evil lurks, waiting to take anyone and everyone who is willing to follow this path. It isn't something reserved for one religion, region, or people, although it foments when we don't love God or neighbor as Jesus directs us to do. If we rely on the same tools of destruction or replace our God-given hopes with the despair or anger of our enemies, we will most certainly forget the power of evil does not match the power of our God. We will lose ourselves and we will lose our neighbors.
May we never forget.