I don't see why the date couldn't have been chosen months in advance, and probably was.
When I am planning a trip abroad, I make my airline reservations first before I make other detailed plans.
The terrorists may have set an optimum date. But they could not have committed to it until a few days before the event.
For the plan to succeed, it was absolutely necessary that the weather over the Eastern seaboard be clear. There was no point in hijacking the planes if they couldn't navigate them to their targets visually.
As it happened, September 11 was a picture perfect day, with virtually unlimited visibility all across the northeastern quarter of the country.