summer solstice vs June 21

summer solstice and June 21 both are nouns.

Word	Noun	Adjective	Verb	Adverb
summer solstice	Yes	No	No	No
June 21	Yes	No	No	No

As nouns, summer solstice and June 21 are synonyms defined as:

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

Other synonyms of summer solstice include midsummer.

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