|I say "Have a good TWO!"|
That means day AND night.
Could it be that people say “Have a good one” so they don’t have to decide between saying “Have a good day” and “Have a good night”? It’s not difficult. If it’s light out you say “Have a good day” if it’s dark out you say “Have a good night”. Saying “Have a good one” sounds like you really don’t give a damn. You leave the meaning up to them and that’s just lazy. Having a good one doesn’t include having to figure out what that “good one” is. You’re giving the person extra work to interpret your vague statement. That’s not nice.
You should just tell a person to have a good day. When you simply say “Have a good one” they might take it to mean “Have a good minute” and then when that minute is up they’ll have a horrible accident. Give them a specific amount of time for goodness. Give them the whole day or don’t bother.