Twenty years after hijacked airliners smashed into New York City's World Trade Centre and the Pentagon outside Washington, Americans will honour the nearly 3,000 lives lost on Sept 11, 2001, and reflect on how the attacks have shaped the country's view of the world and itself.
The centrepiece of Saturday's events is a visit by US President Joe Biden to the three sites associated with the attacks. He will go to ground zero in lower Manhattan; to the Pentagon in Arlington, Virginia; and to Shanksville, Pennsylvania, where United Airlines Flight 93 crashed after passengers tried to regain control of the hijacked plane, reports Reuters.
The remembrances have become an annual tradition, but Saturday takes on special significance, coming 20 years after the morning that many views as a turning point in American history, a day that gave Americans a sense of vulnerability that has deeply influenced the country's political life since then.
In a painful reminder of those changes, only weeks ago US and allied forces completed a chaotic withdrawal from the war the United States started in Afghanistan in retaliation for the attacks - which became the longest war in US history. And the COVID-19 pandemic, which so far has claimed more than 655,000 lives in the United States, continues.
Clifford Chanin, executive vice president at the National September 11 Memorial & Museum built at the site of the World Trade Centre attack, said the two-decade milestone would serve as a "moment of high emotion" for the country, a time to consider "where we've been and where we are headed."
"Of course, we are in the middle of another unimaginable event right now with the COVID pandemic, but if 9/11 brings us anything in terms of what happened here and at the other attack sites, it is a message of resilience," Chanin told reporters this week.
In New York City, the ceremony at the Sept. 11 Memorial will begin with a moment of silence at 8:46 a.m. EDT (1246 GMT), the exact time when the first plane flew into one of the World Trade Centre's twin towers. After that, family members will recite the names of 2,977 victims, an annual ritual that will last four hours.
At sunset on Saturday, 88 powerful lightbulbs will project twin beams four miles (6.4 km) into the sky to mirror the shape of the fallen towers. This year, buildings throughout Manhattan, including the Empire State Building and Lincoln Centre Plaza, will join the commemoration by illuminating their facades in blue.
'MY HEART BREAKS'
The 20-year milestone arrives as political leaders and educators fret over the thinning collective memory of that day. Some 75 million Americans - equal to nearly a quarter of the estimated U.S. population - have been born since Sept 11, 2001.
For some, the tumultuous events in Afghanistan have compounded the psychological toll of the day, raising questions about whether the US military's mission there was in vain.
"I love America and my fellow Americans, but I am ashamed about how we are handling our exit and my heart breaks for those whose lives have been lost or destroyed by our actions," said Wells Noonan, whose brother Robby was in the North Tower.
Noonan said she would spend Saturday morning at a ceremony in her hometown of Greenwich, Connecticut, to honour 33 people with ties to the New York City suburb who were killed, before returning home to be with family and reminisce "about the days with Robby."
SUBWAY SERIES
To mark the anniversary, the New York Mets and New York Yankees professional baseball teams will play each other on Saturday as part of a special Subway Series, their first game on Sept 11 since the attacks.
Baseball was an important symbol of New York resilience in the aftermath of 9/11 as the city struggled to come to grips with the shock, grief and physical destruction of the attacks.
Ten days after the attacks, the Mets become the city's first pro sports team to return to the field, playing the Atlanta Braves. The game, in which the Mets won 3-2, provided a healing diversion for the scarred city and a signal that it was ready to begin moving forward again.
On Saturday, former Mets manager Bobby Valentine will throw out the first pitch to former New York Yankees manager Joe Torre. The players will wear caps bearing logos for the New York City Fire Department and other first responders during the game.
AROUND THE COUNTRY
While many of the large events will happen in and around New York City, people across the country have planned events to remember those who died and to educate the public.
At the Pentagon, headquarters of the US Defence Department, an American flag will be unfurled on the west side where an aeroplane hit the building at precisely 9:37 a.m. EDT (1337 GMT on Sept 11, 2001. Later, the department will hold a private ceremony to honour the 184 people killed there.
In Shanksville in southwestern Pennsylvania, family and guests will gather at the National Memorial there to honour the 40 people killed when United Airlines Flight 93 crashed in a farm field.
In Houston, people will gather on Saturday for the "9/11 Heroes Run." At a US Navy training installation outside Chicago, 2,977 flags have been placed in a field to honour each of those killed in the attacks 20 years ago.