Sun vs day of rest

Sun and day of rest both are nouns.

Word	Noun	Adjective	Verb	Adverb
Sun	Yes	No	No	No
day of rest	Yes	No	No	No

As nouns, day of rest is a hypernym of Sun; that is, day of rest is a word with a broader meaning than Sun:

Sun: first day of the week; observed as a day of rest and worship by most Christians
day of rest: a day set aside for rest

Other hypernyms of Sun include rest day.

Sun (noun)	day of rest (noun)
the star that is the source of light and heat for the planets in the solar system	a day set aside for rest
first day of the week; observed as a day of rest and worship by most Christians