The implication of "turn the mattress"

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?

Solution 1:

Even today, if you buy a mattress, you'll find instructions telling you to turn it on a regular basis, so that you wear out each end, and each side, evenly. Not daily, but every couple of months.

Turning the mattress would have been one of the duties of a housemaid. With it being a fairly physical job, and one that nobody's likely to notice if you don't do it, it's one of the first jobs that a lazy worker would skip.

Hence, by saying that this housemaid turned the mattress every day, she is saying that the girl is conscientious and reliable.

Edit: I discovered in this list of old superstitions:

When you have occasion to turn the mattress of a bed remember that if you turn it from foot to head, you'll never wed, and also that this task should never be performed on a Friday.

Solution 2:

No, it means exactly what it says. It is advisable to turn a mattress over from time to time to ensure that it gets worn evenly. Mrs Hargreaves clearly thinks that not all housemaids can be relied upon to do this, although hers was an exception. I’m not sure, however, of the relevance of not doing it on Friday.