Sun vs Lord's Day

Sun and Lord's Day both are nouns.

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

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

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

Other synonyms of Sun include Dominicus, Sunday.

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