The following sentences are from Agatha Christie:
She had at one time been their housemaid. Such a nice girl, Mrs. Hargraves always said–thoroughly to be relied upon to turn the mattresses every day―exept Friday, of course.
Why if a girl (housemaid) is a very nice girl, is she relied upon to turn the mattresses?
Are mattresses private?
Does this expression ("turn the mattresses") have some implication or connotation?