All 5 Uses

9/11
in
Nineteen Minutes
(Edited)

It was why these schools, post-9/11, had teachers wearing ID all the time and doors locked during the day-the enemy was always supposed to be an outsider, not the kid who was sitting right next to you.
Chpt 1

9/11 = September 11, 2001; when suicide bombers hijacked US airliners and used them as missiles to kill about 3,000 people
A thought occurred to him: that ticker tape hadn't existed until 9/11-until people were so scared that they needed to know, without any delay, the facts of the world they inhabited.
Chpt 1
Part of going back to normal meant erasing the boundaries of what was abnormal, and within a few months, the way Alex had felt on 9/11 was slowly forgotten, like a tide washing out a message she'd once scrawled on the sand.
Chpt 1
Life was a series of ifs-a very different outcome if you'd only played the lottery last night; if you had picked a different college; if you had invested in stocks instead of bonds; if you had not been taking your kindergartner to his first day of school the morning of 9/11.
Chpt 2
Is it really fair to compare bullying to the trauma seen by war veterans in Iraq, or survivors from 9/11?
Chpt 2 *

Definitions:

(1) (9/11)
the terrorist attacks on the United States on September 11, 2001, when hijacked passenger planes were flown into the World Trade Center and the Pentagon, killing nearly 3,000 people

On September 11, 2001, 19 terrorists from the group al-Qaeda hijacked four U.S. passenger airplanes. Two were crashed into the twin towers of the World Trade Center in New York City, causing both towers to collapse. A third plane hit the Pentagon. The fourth plane, crashed into a field in Pennsylvania after passengers fought back. Almost 3,000 people were killed, most of them civilians and first responders. The attacks led to major changes in U.S. security policies and to long wars in Afghanistan and Iraq.
(2) (meaning too rare to warrant focus)
Much less commonly, 9/11 can refer to any September 11th.