Midsummer's Day vs St John's Day

Midsummer's Day and St John's Day both are nouns.

Word	Noun	Adjective	Verb	Adverb
Midsummer's Day	Yes	No	No	No
St John's Day	Yes	No	No	No

As nouns, Midsummer's Day and St John's Day are synonyms defined as:

Midsummer's Day and St John's Day: a quarter day in England, Wales, and Ireland

Other synonyms of Midsummer's Day include June 24, Midsummer Day.

Midsummer's Day (noun)	St John's Day (noun)
a quarter day in England, Wales, and Ireland	a quarter day in England, Wales, and Ireland