On Friday, the Boston Police Department announced plans to beef up security during the city’s Fourth of July festivities in the wake of new remarks from Boston bombing suspect Dzhokhar Tsarnaev that he and his brother originally scheduled a bombing attack for Independence Day. The reference has renewed interest in the symbolic scheduling of terrorist strikes against the West.

Unfortunately for counterterrorism officials, the history of attacks against Western targets is a scattered mix of dates ranging from obvious national holidays to obscure events with only the most tangential relationship to the United States. Let’s review some of the known rationales for the scheduling of terror.

Dec. 25

Some dates make sense. When Umar Farouk Abdulmutallab, a Nigerian with concealed plastic explosives in his underwear, attempted to blow up a passenger flight to Detroit on Christmas day, Anwar al-Awlaki , a Yemen-based al Qaeda cleric accused of orchestrating the plot, issued a statement to the American people describing the rationale of the strike on "the holiest and most sacred days to you, Christmas Day." Given that between 73 and 76 percent of Americans identify as Christian, the date is logical (if there can be a logic to killing innocent civilians). But other dates have less of a tie-in to the United States.

Sept. 11

Despite the widely circulated myth that al Qaeda selected the date 9/11 for its similarity to the emergency call number 9-1-1, the date was important to the terrorist network because of its relationship with Islam. As Lawrence Wright wrote in his Pulitzer Prize-winning book The Looming Tower , on Sept. 11, 1683, the King of Poland launched the battle that turned back the advance of Muslim armies. "For the next three hundred years, Islam would be overshadowed by the growth of Western Christian societies," Wright explained. Osama bin Laden saw the attack on the World Trade Center as Islam’s big comeback. The date has since been used by other terrorists, including the jihadists who struck the U.S. compound in Benghazi, killing U.S. ambassador Chris Stevens and three other Americans last year.

Feb. 26

Other dates of terrorist attacks reveal how arbitrary the timing of these strikes can be. For instance, the first bombing of the World Trade Center occurred on Feb. 26, 1993, a date that had no significance to any of the parties involved. The attack was originally plotted for Feb. 23 , the day the U.S. ground offensive began in Iraq in 1991. Another reason not to place too much importance on specific dates: Who’s to say terrorists will be punctual?

March 11

Date doesn’t ring a bell? It does to Spaniards, who suffered the 2004 Madrid train bombings attributed to an al Qaeda-inspired terrorist cell. Reports suggest the date was significat because it occurred exactly two and a half years after Sept. 11.

Dec. 31

New Year’s Eve has repeatedly been an aspirational date for terrorist attacks, according to U.S. officials. In 2000, for example, U.S. authorities apprehended Ahmed Ressam at a border crossing in Washington state for carrying bomb-making equipment and plotting to blow up Los Angeles International Airport on New Year’s Eve.

As for the Boston bombing, it’s worth noting that while signs indicate Tamerlan Tsarnaev was influenced by radical Islam, a clear explanation for the motive behind the marathon bombings has not been revealed.?