If there was a set of reasons that September 11, 2001 was chosen to fly a plane into the Pentagon; and if we ever actually discover those reasons: I'd be willing to bet money that 9/11/2001 was chosen because it was the anniversary of ground-breaking for the building. 9/11/1941 was the day they broke ground to begin construction of the Pentagon. It would make sense to try to make a point by attempting to destroy it on that same day.

On the other hand, does it make sense to try to make sense out of something senseless?