September 11 vs Sept. 11

September 11 and Sept. 11 both are nouns.

Word	Noun	Adjective	Verb	Adverb
September 11	Yes	No	No	No
Sept. 11	Yes	No	No	No

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

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

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

September 11 (noun)	Sept. 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