I have been having some really rotten days lately, so I decided to write about this phrase “have a nice day”. 
If you’re having an extremely rotten day, having someone say to you “have a nice day” can really be a major drag.
I don’t tell you what to do or feel, so why should you tell me how I should think or feel? The same goes for stuff like “have a great time” — what happens if I don’t?
If you even consider using a phrase like this, then be very careful that you use it with this caveat: if there is any doubt whatsoever, it is best not to tell someone how they should think or feel.
Besides that, it is a rather empty thing to say — normal people would wish almost anyone (even a terrorist) to have a nice day (after all, who would really want to meet up with a terrorist who wasn’t having a nice day?). In my opinion, it would be better to say something like “I really enjoyed chatting with you” (or something like that) and “I can’t wait for another chance to…” (whatever).
That said: Have a nice day! 
