The September 11 attacks were planned to take place five or six months earlier but were delayed because the ringleader was not ready, according to a leak from the commission investigating the terrorist plot.
http://www.guardian.co.uk/september11/story/0,11209,1239654,00.html

The leak emerged just before the final public hearings by the commission, today and tomorrow, at which the US military is expected to be criticised for its apparent delay in scrambling fighter jets to intercept the hijacked planes.

The leak underlines the confusion in the US government's attempt to unpick the chain of events: it conflicts with FBI accounts, which have repeatedly suggested that the attacks were brought forward after the arrest of Zacarias Moussaoui, the alleged "20th hijacker".

Atta and his accomplices only made reconnaissance flights in May 2001, the FBI has reportedly discovered. Six of them flew individually across the country from the east coast and back that summer.

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Analysts who have pored over the hijackers' computer and e-mail records believe they carefully researched flight plans and schedules, and specifically selected Boeing 757 and 767 jets, on which they had trained. This was more important than the choice of a particular airport.
http://news.independent.co.uk/world/americas/story.jsp?story=531944