Lord's Day vs Sun

Lord's Day and Sun both are nouns.

Word	Noun	Adjective	Verb	Adverb
Lord's Day	Yes	No	No	No
Sun	Yes	No	No	No

As nouns, Lord's Day and Sun are synonyms defined as:

Lord's Day and Sun: first day of the week; observed as a day of rest and worship by most Christians

Other synonyms of Lord's Day include Dominicus, Sunday.

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