Sabbath vs day of rest

Sabbath and day of rest both are nouns.

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

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

Sabbath: a day of rest and worship: Sunday for most Christians; Saturday for the Jews and a few Christians; Friday for Muslims
day of rest: a day set aside for rest

Other hypernyms of Sabbath include rest day.

Sabbath (noun)	day of rest (noun)
a day of rest and worship: Sunday for most Christians; Saturday for the Jews and a few Christians; Friday for Muslims	a day set aside for rest