All 3 Uses

9/11
in
No Easy Day
(Auto-generated)

This was 2007, and 9/11 was six years behind us.†
p. 150.8

9/11 = September 11, 2001; when suicide bombers hijacked US airliners and used them as missiles to kill about 3,000 people
Al-Qahtani eventually told them that Khalid Sheikh Mohammed, the planner of the September 11 attacks, sent him to the United States.†
p. 164.6 *

September 11 attacks = terrorist attacks on the U.S. on September 11, 2001, using hijacked planes to kill nearly 3,000 people
And around the globe, we worked with our friends and allies to capture or kill scores of al Qaeda terrorists, including several who were a part of the 9/11 plot," Obama said.†
p. 274.9

9/11 = September 11, 2001; when suicide bombers hijacked US airliners and used them as missiles to kill about 3,000 people

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.