Wiktionary
have a good one
vb. 1 (context idiomatic English) To enjoy an event or occasion, especially a holiday or one's birthday. 2 formula used when parting.
Usage examples of "have a good one".
If I had any Kitty-mouse I'd have a good one who liked you to play in safe pleasant ways, and not destroy and frighten.
They all do things with reasons, and as long as I have a good one, theyll believe me.