Guy/Girl Friday

A modern day expression? This is usually seen in the claasified ads under "help wanted". What are they looking for. Is it an all around office helper?
Why Friday?

From Daniel Defoe's novel entitled "Robinson Crusoe":

"In a little time I began to speak to him, and teach him to speak to me; and,first,I made him know his name should be Friday, which was the day
I saved his life."

A guy or girl friday is, as you mentioned, an all around office helper just as Friday was Robinson's helper and guide in the novel.