Sept. 11 vs September 11

Sept. 11 and September 11 both are nouns.

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

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

Sept. 11 and September 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, 9/11, Sep 11, September 11 attacks.

Sept. 11 (noun)	September 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