black_paws wrote in linguaphiles Puzzled London

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"a day" and "per day"

When is it correct to say "a day" and when is it correct to say "per day" (pardon my prescriptive way of asking)? How interchangeable are they?

I have a rough idea but any rules governing this would be very useful.
I've already tried googling but the first result came up as "calories per day calculator".

Very grateful, IOU, etc.