holy day vs Dormition

holy day and Dormition both are nouns.

Word	Noun	Adjective	Verb	Adverb
holy day	Yes	No	No	No
Dormition	Yes	No	No	No

As nouns, Dormition is a hyponym of holy day; that is, Dormition is a word with a more specific, narrower meaning than holy day:

holy day: a day specified for religious observance
Dormition: celebration in the Eastern Orthodox Church of the Virgin Mary's being taken up into heaven when her earthly life ended; corresponds to the Assumption in the Roman Catholic Church and is also celebrated on August 15th

Other hyponyms of holy day include fast day, Christian holy day, Jewish holy day, Feast of Dormition.

holy day (noun)	Dormition (noun)
a day specified for religious observance	celebration in the Eastern Orthodox Church of the Virgin Mary's being taken up into heaven when her earthly life ended; corresponds to the Assumption in the Roman Catholic Church and is also celebrated on August 15th