midsummer vs summer solstice

midsummer and summer solstice both are nouns.

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

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

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

Other synonyms of midsummer include June 21.

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