I would take it as the day of the event that started a small revolution, that revolution being the end of international terrorism. That day got several peoples heads out of their asses. Before that day, terrorism was tolerated and ignored, and it was more or less considered "politicaly incorrect" to attempt to bring down any terrorist group.
If you recall, the Clinton administration had a perfect oportunity to bring down bin ladin. They didn't move in fear of it causing an international incident. September 11th tought everybody that merely allowing these groups to exist is just asking for trouble.