Sunday vs Sun

Sunday and Sun both are nouns.

Word	Noun	Adjective	Verb	Adverb
Sunday	Yes	No	No	No
Sun	Yes	No	No	No

As nouns, Sunday and Sun are synonyms defined as:

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

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

Sunday (noun)	Sun (noun)
United States evangelist (1862-1935)	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