Sun vs funday

Sun and funday both are nouns.

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

As nouns, funday is a hyponym of Sun; that is, funday is a word with a more specific, narrower meaning than Sun:

Sun: first day of the week; observed as a day of rest and worship by most Christians
funday: Sunday as a day for relaxing and having fun

Sun (noun)	funday (noun)
the star that is the source of light and heat for the planets in the solar system	Sunday as a day for relaxing and having fun
first day of the week; observed as a day of rest and worship by most Christians