Leaving a restaurant today and the hostess says, "Have a good one." I turned my daughter and asked - "a good what? A good day? Why not just say that then? It's the same amount of syllables and effort. If it is a period of time - why wish me just a good day - wouldn't a good week be better? How about wishing me a good year?"
If it a random saying and word - then why not say "have a good pizza?" or "have a good cable box?"
If it is just numerical in basis - then why not wish me a good seven? That would be several times better than a good one.
This was the conversation I had with my daughter leaving the restaurant at lunch.