On Friday the U.S. State Department issued a worldwide travel alert because of an unspecified al Qaeda threat. The location of that threat, the department said in a bulletin, is "particularly in the Middle East and North Africa, and possibly occurring in or emanating from the Arabian Peninsula." As a result, an unprecedented 21 embassies and consulates in 17 countries in the Middle East, North Africa and South Asia will close on Sunday.
Sunday is also the 27th day of Ramadan and a particularly holy day for the world's Muslims as it is the "Night of Power," when the first verses of the Koran were revealed to the Prophet Mohammed.
It is also seen by al Qaeda's would-be martyrs as a particularly auspicious day to die.
On the Night of Power in 2000, which that year fell on January 3, al Qaeda militants attempted to launch a suicide attack against the American warship USS The Sullivans off the coast of Yemen with a bomb-filled boat.
That attack failed, but the same group of militants then attacked the USS Cole 10 months later, again using the tactic of a bomb-laden boat, which exploded, killing 17 American sailors.
It was the day after the Night of Power in December 2001 that Osama bin Laden signed his will as he feared death from American bombs falling during the battle of Tora Bora in eastern Afghanistan.
Al Qaeda and aligned organizations have a long history of attacking U.S. embassies and consulates, beginning in 1998 with the bombings of the embassies in Tanzania and Kenya, which killed more than 200, and including, most recently, the storming of the U.S. government facility in Benghazi, Libya, on the 11th anniversary of 9/11 last year.
For al Qaeda, these diplomatic compounds are attractive targets because they symbolize American power and because their locations are widely known. Indeed, several of the embassies and consulates that are being closed on Sunday have been attacked by al Qaeda and affiliated groups before.
In Saudi Arabia, al Qaeda terrorists stormed the U.S. Embassy compound in Riyadh on May 12, 2003, killing 36 people. A year-and-a-half later, militants breached the outer wall of the U.S. consulate in Jeddah and opened fire on the staff, killing five.
The U.S. embassy in Sana'a, Yemen, was targeted by an al Qaeda affiliate two times in 2008, attacks that killed 21.
On September 13, 2011, the Haqqani network, a Taliban group closely associated with al Qaeda, attacked the U.S. Embassy in Kabul, killing five Afghan police officers and 11 civilians.
Though U.S. officials have not confirmed the targeting of a specific embassy, it is likely that the American compound in Cairo is a particular point of concern. In May, three men were arrested by Egyptian officials, who said the men were planning to attack the embassy. Officials said they discovered 22 pounds of aluminum nitrate, instructions on how to make bombs, and materials published by al Qaeda in the Islamic Maghreb, the network's North African affiliate.
Another possible factor in the timing of the al Qaeda alert is that in the past two weeks, massive jailbreaks in Iraq and Libya have released more than a thousand prisoners, some with significant ties to al Qaeda. Very few of those inmates have been recaptured.
The U.S. embassies in Baghdad, Iraq, and Tripoli, Libya, are among the facilities that will be closed on Sunday.
Peter Bergen is CNN's national security analyst, a director at the New America Foundation and the author of Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad. Bailey Cahall is a research associate at the New America Foundation.