Often that means an anniversary date or holiday.
Whether it's casting a pall over Christmas or forcing the public to remember an event that drove the terrorist to violence, datebook terrorism is all about proving who controls the agenda, said Joan Deppa, a Syracuse University communications professor.
"They're saying, 'I've got your attention, and by the way, if I can make your holiday miserable, all the better,' " she said.
Homeland Security Director Tom Ridge said the closeness this year of Christmas and Hanukkah to Eid al-Fitr, the joyous three-day feast that ends the holy month of Ramadan, could be tempting to terrorists who have a history of striking during religious observances.
"We do know that the next several weeks, which bring the final weeks of Ramadan and important religious observations in other faiths, have been times when terrorists have planned attacks in the past," he said.
Ridge's warning stemmed from multiple, nonspecific threats for this period, but he made it clear that the calendar figured high in the government's concerns.
He cited the first World Trade Center attack in 1993, which fell during Ramadan, and a foiled series of attacks planned around New Year's Day 2000, timed both for Ramadan and millennial celebrations. In recent years, Eid al-Fitr has been marked by violence in Indonesia in 1999, and in Bahrain, Egypt and Israel in 1996.
As familiar as datebook terror may be in the Middle East, it's not just Islamist militants who time their attacks.
Timothy McVeigh parked a truck bomb outside a federal building in Oklahoma City on April 19, 1995, and killed 168 people - retribution, he said, for the deadly FBI raid on an encampment near Waco, Texas, exactly two years earlier. More than 80 people died then, people McVeigh considered martyrs to his anti-government creed.
In some countries, citizens could count on attacks according to the calendar. Until Peruvian authorities gutted the Shining Path in the early 1990s, the Maoist insurgency regularly staged car bombings and other attacks on the birthdays of founder Abimael Guzman (Dec. 3) and his idol, Mao Tse-tung (Dec. 26).
Before a 1997 truce, the history-obsessed Irish Republican Army would often stage attacks on dates marked by its followers but utterly obscure to others; for instance, Aug. 9, the anniversary of the introduction of iternment without trial in Northern Ireland in 1971.
Some attacks are timed to deprive the "oppressor" of joys denied the terrorist, and those he may champion.
An IRA car bomb detonated outside Harrod's department store during peak Christmas shopping in 1983 killed six people, embittering Christmas for the British and reassuring jailed IRA terrorists that their cause was alive.
"These are acts of mass communication, but often aimed at more than one audience," Deppa said. "Different audiences will get different messages."
That was the case on Feb. 25, 1994, when Israeli settler Baruch Goldstein shot and killed 29 Muslim worshippers at a Hebron sanctuary holy to Jews and Muslims before he was beaten to death.
Eid al-Fitr and the Jewish holiday of Purim coincided that year, and Goldstein was furious that Muslim celebrations at the ancient Cave of the Patriarchs were likely to disrupt prayer sessions marking the Jewish holiday.
Hebron's Muslim community was plunged into mourning, while some Jewish settlers in Hebron and neighboring Kiryat Arba celebrated Goldstein as a martyr.
Two years later, Islamic militants staged four suicide bomb attacks in Israel around the same time of year, killing 57 - including many children outfitted in their Purim costumes.
America's worst day of terrorism has no prior significance on the calendar so far as anyone knows. Sept. 11 created its own anniversary.
By Ron Kampeas © MMI The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed