Six Day War vs Six-Day War

Six Day War and Six-Day War both are nouns.

Word	Noun	Adjective	Verb	Adverb
Six Day War	Yes	No	No	No
Six-Day War	Yes	No	No	No

As nouns, Six Day War and Six-Day War are synonyms defined as:

Six Day War and Six-Day War: tension between Arabs and Israeli erupted into a brief war in June 1967; Israel emerged as a major power in the Middle East

Other synonyms of Six Day War include Arab-Israeli War.

Six Day War (noun)	Six-Day War (noun)
tension between Arabs and Israeli erupted into a brief war in June 1967; Israel emerged as a major power in the Middle East	tension between Arabs and Israeli erupted into a brief war in June 1967; Israel emerged as a major power in the Middle East