(CNN) – Fifteen years ago Sunday, the United States changed forever when four airliners crashed into the Twin Towers in New York City, the Pentagon in Washington, D.C., and a field outside Shanksville, Pennsylvania.
The age of terror had begun.
"September 11, 2001, touched every single one of us," New York City Mayor Bill de Blasio said Saturday during a memorial service at St. Patrick's Cathedral for the city's fire department. "There is no New Yorker who somehow evaded the pain of that day. We all felt it. We all were affected. Everyone felt it. Everyone suffered."
The 2,977 men and women who died in the attacks are being remembered this weekend with parades and memorials throughout the country. Special services will be held at the spots where the planes crashed.
Here's a timeline of Sunday's events.
8:46 a.m ET: A moment of silence will be observed at Ground Zero, where the towers stood, to mark the time the first plane hit the North Tower.
Houses of worship throughout New York City will toll their bells, and families of victims of the 2001 attacks -- as well as the 1993 bombing at the World Trade Center -- will read the names of the people killed. A livestream of this event can be seen at www.911memorial.org.
At the same time, a moment of silence will be observed at the White House.
9:03 a.m. ET: A moment of silence will be observed at the time the second plane struck the South Tower. Afterward, the reading of the victims' names will resume.
9:30 a.m. ET: A ceremony will begin at the Pentagon. It can be watched live on the Department of Defense website.
9:37 a.m ET: A moment of silence will mark the time Flight 77 struck the Pentagon. The reading of the names resumes.
In Washington, President Barack Obama, Defense Secretary Ash Carter and Joint Chiefs of Staff Chairman Marine Corps Gen. Joe Dunford will address the families and friends of the 9/11 victims in a ceremony at the Pentagon Memorial.
9:45 a.m. ET: A service will be held at the Flight 93 National Memorial in Pennsylvania.
9:59 a.m. ET: A moment of silence will mark the time of the fall of the South Tower. The reading of names continues.
10:03 a.m. ET: Moment of silence to mark the time hijacked Flight 93 crashed outside Shanksville, Pennsylvania, short of their intended target after the passengers and crew fought back against the terrorists.
10:28 a.m. ET: Moment of silence to observe the time of the fall of the North Tower. The reading of names continues.
12:30 p.m. ET: The ceremony ends.
Presidential candidates Hillary Clinton and Donald Trump arrived at Ground Zero on Sunday for the ceremony.
After the September 11 Commemoration Ceremony, New York Governor Andrew M. Cuomo will join more than 450 motorcycle riders participating in the 9/11 Memorial Motorcycle Ride at a lunch in New York City. The ride pays tribute to the strength, courage and heroism of the first responders who led recovery efforts during and following the terrorist attacks.