Sept. 11 vs Sep 11

Sept. 11 and Sep 11 both are nouns.

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

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

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

Sept. 11 (noun)	Sep 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