Sept. 11 vs 9/11

Sept. 11 and 9/11 both are nouns.

Word	Noun	Adjective	Verb	Adverb
Sept. 11	Yes	No	No	No
9/11	Yes	No	No	No

As nouns, Sept. 11 and 9/11 are synonyms defined as:

Sept. 11 and 9/11: the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs

Other synonyms of Sept. 11 include 9-11, Sep 11, September 11, September 11 attacks.

Sept. 11 (noun)	9/11 (noun)
the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs	the day in 2001 when Arab suicide bombers hijacked United States airliners and used them as bombs