Sun vs Sunday

Sun and Sunday both are nouns.

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

As nouns, Sun and Sunday are synonyms defined as:

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

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

Sun (noun)	Sunday (noun)
the star that is the source of light and heat for the planets in the solar system	United States evangelist (1862-1935)
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