summer solstice vs midsummer

summer solstice and midsummer both are nouns.

Word	Noun	Adjective	Verb	Adverb
summer solstice	Yes	No	No	No
midsummer	Yes	No	No	No

As nouns, summer solstice and midsummer are synonyms defined as:

summer solstice and midsummer: June 21, when the sun is at its northernmost point

Other synonyms of summer solstice include June 21.

summer solstice (noun)	midsummer (noun)
June 21, when the sun is at its northernmost point	June 21, when the sun is at its northernmost point