9/119/11 /ˌnaɪn ɪˈlevən/ noun [uncountable] 
September 11, 2001, when terrorists used planes to attack New York and Washington: 
the terrifying events of 9/11 a 9/11 disaster relief fund
September 11, 2001, when terrorists used planes to attack New York and Washington: the terrifying events of 9/11 a 9/11 disaster relief fund