Most anniversaries are cause for celebration — like birthdays or a 50th wedding anniversary. Holidays celebrate the founding of a nation or the end of a war or the birthday of a hero.
But sometimes an anniversary is a reminder of something we would prefer to forget.
Sunday marks the 21st anniversary of the terrorist attacks against the United States—planned and launched by al Qaeda from Afghanistan—that killed 2,977 innocent people.
September 11, 2001, was one of the darkest days in US history. Former President George W. Bush declared that "terrorist attacks can shake the foundation of our tallest buildings, but they cannot touch the foundation of America."
Much has changed since then, but following the disastrous U.S. military withdrawal last year, the Taliban once again rule Afghanistan, and al Qaeda enjoys a safe haven there—just as it did on Sept. 11, 2001.
Biden is the fourth president to console the nation on the anniversary of that dark day, one that has shaped many of the most consequential domestic and foreign policy decisions made by the chief executives over the past two decades.
The terror attack defined the presidency of George W. Bush, who was reading a book to Florida schoolchildren when the planes slammed into the World Trade Center.
He spent that day being kept out of Washington for security reasons - a decision that then-Sen. Biden urged him to reconsider, the current president has written - and then delivered a brief, halting speech that night from the White House to a terrified nation.
The following year, Bush chose Ellis Island as the location to deliver his first anniversary address, the Statue of Liberty over his shoulder as he vowed, "What our enemies have begun, we will finish." The wars in Iraq and Afghanistan were still deadly when Obama visited the Pentagon to mark his first Sept. 11 in office in 2009.
"No words can ease the ache of your hearts," he said.
"We recall the beauty and meaning of their lives," he said. "No passage of time, no dark skies can dull the meaning of that moment."
By the time Obama spoke at the 10th anniversary, attack mastermind Osama bin Laden was dead, killed in a May 2011 Navy SEAL raid. Though the nation remained entangled overseas, and vigilant against terror threats, the anniversary became more about healing.
President Donald Trump pledged to get the U.S. out of Afghanistan, but his words during his first Sept. 11 anniversary ceremony in 2017 were a vivid warning to terrorists, telling "these savage killers that there is no dark corner beyond our reach, no sanctuary beyond our grasp, and nowhere to hide anywhere on this very large earth."
On Saturday, as Biden was making his way to all three sites, Bush was to pay his respects in Shanksville. Trump planned at least one stop in Manhattan and was to deliver ringside commentary at a boxing match at a casino in Hollywood, Florida.