Concerning the date September 11 itself, I think the most likely explanation is that this is the pre-Islamic Egyptian new year celebrated by Egyptian (Coptic) Christians. According to his handler Ramzi Binalshibh, the date was proposed by Atta himself... The modern ideology of jihad, as formulated by Sayyid Qutb (another Egyptian), says that modern Islamic societies have slipped back into "jahiliyah", pre-Islamic ignorance of God, and their rulers are therefore legitimate targets. The Islamists also say that Judaism and Christianity started out in truth but have become corrupted (this is how they can claim Abraham and Jesus as Muslim prophets). The Copts say that September 11 commemorates Christians who were martyred by the pagan Roman emperor Diocletian, but the date does also have a resonance with truly ancient Egyptian culture (the equinoctial flooding of the Nile, the rise and fall of Sirius in the sky). It also appears to be given some prominence in modern, secular-nationalist Egypt (the current Egyptian constitution was adopted on 11 September 1971, less than a year after Sadat took over from Nasser). Finally, it was an Egyptian Copt (Boutros-Boutros Ghali) who was UN Secretary-General at the high point of the post-Gulf War "peace process"... I don't think there's anything very deep here, I think it was opportunistic - Atta had to choose a date, and September 11 fell within range, giving him a chance to send a symbolic message specifically to his countrymen, on top of everything else.