I think when people say that they mean: "Remember the day poor old good-for-the-world 'America' was victimized out of the blue by people who hate our way of life, our freedom, our Constitution, our Declaration of Independence, and our Founding Fathers." That, frankly, is bullshit. A herculean ignorance of recent history or nationalistic self-blindness is required to see things that way. Anyone who says 9/11 shows the United States (i.e., the government) cannot practice noninterventionism in foreign affairs in this dangerous world has no idea what he is talking about. (For details see this and this.)
September 11 should be remembered as the day the biggest flock of chickens, to date, came home to roost. Blowback Day. A better slogan would be: Never Again. And the way to make that come true is to heed George Washington:
Against the insidious wiles of foreign influence (I conjure you to believe me, fellow-citizens) the jealousy of a free people ought to be constantly awake, since history and experience prove that foreign influence is one of the most baneful foes of republican government. But that jealousy to be useful must be impartial; else it becomes the instrument of the very influence to be avoided, instead of a defense against it. Excessive partiality for one foreign nation and excessive dislike of another cause those whom they actuate to see danger only on one side, and serve to veil and even second the arts of influence on the other. Real patriots who may resist the intrigues of the favorite are liable to become suspected and odious, while its tools and dupes usurp the applause and confidence of the people, to surrender their interests.
The great rule of conduct for us in regard to foreign nations is in extending our commercial relations, to have with them as little political connection as possible....
It is our true policy to steer clear of permanent alliances with any portion of the foreign world;... Harmony, liberal intercourse with all nations, are recommended by policy, humanity, and interest.