what does Cuando es la fiesta de Arturo mean? and how do you answer it in a complete sentence.

"Cuando es la fiesta de Arturo"

-When is Arturo's party?

If you want to answer in spanish (don't know my accents) you would say:

-La fiesta de Arturo es ________

And then write when it is.

"¿Cuándo es la fiesta de Arturo?" is a question in Spanish, which translates to "When is Arturo's party?" in English.

To answer this question in a complete sentence, you could say:
"La fiesta de Arturo es el (insert date) a las (insert time)."
This translates to "Arturo's party is on (insert date) at (insert time)." Fill in the date and time with the actual details of Arturo's party.