I would say that since they needed to make it difficult for us to use the Northern Alliance to retaliate against bin Laden - and because the Taliban wanted to expand into the Central Republics- that the hijackings and the assassination of Northern Alliance leader Masood would have to occur at the most advantageous time for a Taliban all-out offensive against the Northern Alliance. This had to narrow the window somewhat. [If the US had reacted a bit slower after 911 the Taliban may had planned on denying us bases to Afghanistan's north.]
The hijacking plotters for the US attack would have to select a weekday during business hours to insure the maximum number of casualties at the WTC towers and the Pentagon.
That doesn't limit their options much, but it does limit choices a little more.
Since they were going to try for the White House and failing that, choose the Capitol, then they had to select a date where Congress was in session AND a day on which the President would be in the White House.
Now it gets tough...
They couldn't be too choosy over the locations from which their flights would leave. They had to take what they could get. They also had to take flights that used the type of aircraft on which they had trained.
Now it's REAL tough.
How to insure that bin Laden's favorite target and incidentally, the same target Saddam Hussein's kid's newspaper had referred to in its "Frank Sinatra" & "tunnels of the White House" article that summer... would be worth it, with the Pres and possibly even the VP there in the event it could be hit?
We all know Bush was in Florida that morning, so what did they expect to hit except a white building with Cheney in it?
There was a story about a curious incident that morning in Florida where some ME types pretended to have a scheduled interview with the Pres at his hotel before he went on to the school- they were turned away. To bad the Northern Alliance's Masood did n't turn away reporters when he had the chance.
But ... how many know Bush was scheduled to return to the White House that morning for a meeting and not go visit some other Florida locale? There was a meeting scheduled for him to attend. Who would know about it and its timing better than one or more of the attendees, some of whom would probably like nothing better than to see Bush's face when the WTC towers & Pentagon would be burning?
Why didn't Bush attend the meeting, whose attendee's were in Washington ready to go in?
One reason was that Bush was on Air Force One and AF1 was taking evasive action while the situation was still not fully known and aircraft were still trying to land on command.
We all know about the phoned-in message to the White House about the codes of Air Force One that morning. How did the caller know the info needed to make that threat?
Why threaten AF 1? To get it to land in DC right away?
Other reasons to schedule that date may depend on the need to coordinate with the second wave against the West Coast, for which some suspects had tickets.
Yet another reason could be to coordinate with the anthrax attack- depending on when the perp[s] obtained the anthrax.
I doubt very much that the date itself has significance to the plot since there were already so many requirments to be met- I'm sure muslims could subscribe importance to an date just as they label every former church, city, or other desirable property "one of the Holiest Sites in Islam." Bin Laden knew it would have significance all its own before the sun set that day, whether it was an anniversary of something or not.
There also could have been a need to coordinate with Ansar al Islam's assassinations of Kurdish leaders in Iraq.
Remember the 2001 calender that in the ME (Egypt IIRC) that had picture of the NY syline and a firery plane crashing into the harbor?
2001 summer : (EGYPTIAN-BORN US POSTAL WORKER AHMED ABDEL SATTAR'S CONVERSATION ABOUT HIS 'LETTER OF INTRODUCTION 'TO AFGHAN NORTHERN ALLIANCE LEADER AHMED SHAH MASSOUD IS WIRETAPPED; SATTAR ACTS AS IMPRISONED SHEIKH OMAR ABDEL RAHMAN'S 'COMMUNICATIONS CENTER') But a conversation in the summer of 2001 about the letter surfaced during a wiretap involving Sattar, who [would later be] charged with serving as a communications center for an Egyptian terrorist group allegedly directed by Sheikh Omar Abdel-Rahman from his U.S. prison cell, the Post reported [later]. Abdel-Rahman was convicted in 1995 of plotting to blow up New York landmarks, including the World Trade Center. -- "U.S. man's letter links to Afghan killing," Middle East Times, 2002, http://www.metimes.com/2K2/issue2002-20/eg/us_mans_letter.htm
"Angel is next. . ." Has it ever been resolved (publicly) what the explanation for that is? I saw an interview with Rice in late 2001 confirming the threat was made, but she said they weren't sure if it was a mole or just a leak of a codename that had somehow gotten out.
Good points on the exigencies of the timetable. Trying to hit all those targets at once does limit the options for playing around with the symbolism. I didn't think of correlating it with the Northern Alliance issue, which is also interesting.