They desperately need a symbolic victory as their tactics after 9/11 against America have been a big zero.
The date is symbolic enough. They've made their point about hitting symbolic targets earlier about hitting the Pentagon, and the WTC. What they need this time is more body count, and more new venues.
New York and Washington are already in the game. Al Qaida needs to hit a major city in California (San Fran, San Diego or L.A), Las Vegas or Seattle. Why?
Because they want a war, and they want us to want it. They know the East Coast is on board, because it hit close to home. They know the midwest and the south is on board, because it's a more conservative area that will respond to being attacked. The people who still are not committed are the people out West, because they are both liberal and unaffected by the terror thus far. It's not real to them.
AQ needs to make it real for them, to further their idiotic plans, and I have no doubt they'll try.