solstice vs June 21

solstice and June 21 both are nouns.

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

As nouns, June 21 is a hyponym of solstice; that is, June 21 is a word with a more specific, narrower meaning than solstice:

solstice: either of the two times of the year when the sun is at its greatest distance from the celestial equator
June 21: June 21, when the sun is at its northernmost point

Other hyponyms of solstice include midsummer, summer solstice, winter solstice.

solstice (noun)	June 21 (noun)
either of the two times of the year when the sun is at its greatest distance from the celestial equator	June 21, when the sun is at its northernmost point