On a sunny morning 20 years ago this coming Saturday, history turned in a single hour. Four California-bound commercial airliners, which took off in the northeastern United States on Sept. 11, 2011, were hijacked mid-flight by 19 men from a militant Islamist terrorist group called al-Qaeda (pronounced al-KAY-dah). Two smashed into the World Trade Center twin towers in Lower Manhattan, which collapsed. Another hit the Pentagon military headquarters just outside Washington in Arlington, Va., and the last was forced to crash in rural Pennsylvania when doomed passengers bravely rushed the cockpit. In all, nearly 3,000 people died on a morning that changed our country and the world forever. Nearly half of the victims' remains couldn't be identified.

The 20th anniversary of the attacks will be commemorated at televised ceremonies in Lower Manhattan at the 9/11 Memorial & Museum where the Trade Center was – a site known as Ground Zero. In addition, new documentaries and hours of special programming will be shown on CNN, the History Channel, other cable TV networks and streaming services. Buildings will be bathed in blue lights Saturday across New York and the nation, in solidarity with those marking the somber anniversary.

Houses of worship nationwide will toll their bells at 8:46 a.m. Saturday, the moment when the first jet struck one of the Twin Towers in New York. At the day's main memorial, family members will read aloud the names of those killed, whose names are etched in bronze above pools at a plaza on the attack site. The memorial, normally closed to visitors at 5 p.m., stays open till midnight on the anniversary.

9/11 Memorial & Museum says: "Despite our shared grief in the aftermath of 9/11, hope, resilience, and unity lifted us up as a nation. Twenty years later, these lessons are more important than ever." – Website statement

Columnist says: "Sadly, I wonder: What exactly did we learn from the 9/11 attacks?" – Mike Kelly of NorthJersey.com, who covered the attacks

Historian says: "By the year 2001, the Twin Towers had become universally recognized as a symbol. It represented American engineering know-how. It showed America reaching for the sky. It stood for American capitalism and, with time, America itself. Indeed, that is the reason it was chosen as a target by the terrorists." -- Angus Gillespie, professor of American studies at Rutgers University in New Jersey