Interrogation of captured Islamic terrorists, translation of captured documents, and statistical analysis of their attacks, has revealed what many have long suspected. The big attacks are scheduled for maximum media impact. Not just the international media, but for local consumption as well. For example, although most of Iraq saw little, or no, violence after early 2003, most Iraqis were nervous about the terrorism. That's because the local media reported, in gruesome detail, the attacks that were being carried out in the small part of the country where most of the violence was concentrated. Thus the Islamic terrorists scared many Iraqis they could not reach with their bombs, or gunmen. This backfired a bit for Sunni Arabs living in Shia neighborhoods, as the Shia would respond to the new fear of more Sunni Arab terror bombings, by driving out their peaceful Sunni Arab neighbors.
Counter-terrorism organizations have computer models of terrorist organizations, and these models are believed capable of figuring out, with a high degree of precision, where and when the groups next big attack will take place. Using that information, more resources are applied, and often confirmation of such an attack is obtained, and the attack aborted. Naturally, the inner workings of these models is kept secret, lest the terrorists come up with a way to "work the model" and escape detection.