Lord's Day vs Sunday

Lord's Day and Sunday both are nouns.

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

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

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

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

Lord's Day (noun)	Sunday (noun)
first day of the week; observed as a day of rest and worship by most Christians	United States evangelist (1862-1935)
	first day of the week; observed as a day of rest and worship by most Christians