I was surprised to read in the Washington Post's coverage of the 9/11 report that Bin Laden wanted to execute the plot in response to then-opposition leader Ariel Sharon's visit to the Temple Mount. The Post placed the time of OBL's request as the middle of 2000.
Just to make sure that the Post had the report correct, I checked the report (link also available from the Washington Post). This is the full text of the relevant paragraph:
Bin Ladin had been pressuring KSM for months to advance the attack date. According to KSM, Bin Ladin had even asked that the attacks occur as early as mid-2000, after Israeli opposition party leader Ariel Sharon caused an outcry in the Middle East by visiting a sensitive and contested holy site in Jerusalem that is sacred to both Muslims and Jews. Although Bin Ladin recognized that Atta and the other pilots had only just arrived in the United States to begin their flight training, the al Qaeda leader wanted to punish the United States for supporting Israel. He allegedly told KSM it would be sufficient simply to down the planes and not hit specific targets. KSM withstood this pressure, arguing that the operation would not be successful unless the pilots were fully trained and the hijacking teams were larger.