January 5th, 2011

rabid rabbit

"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.