P.S. "Ten un buen día" sounds very odd to me too. I am not sure, but... quizás lo digan así en Madrid. hehe
jeje, ya empezamos
Bueno, odd it does sound odd, but I cannot see anything wrong with it.
Ten un buen día...sounds like you are giving somebody the order to have a nice day, jeje, but I cannot see a mistake here.
It would be highly unusual, right.
Your suggestions are used in Madrid too, even by Real Madrid fans. jeje
Actually, I have the same problem with the English "Have a nice day." To me, it sounds very much like a command. I keep thinking, What if I don't have a nice day? Is someone going to punish me (beat me up) because I failed to follow his instructions? Will I be forgiven, if I explain that I really tried to have a nice day but was unsuccessful?
I don't think it is given as a command in English, although I have heard others make the same comment you have. It is given as a wish, with the "I hope you" left off, as you pointed out is done is Spanish with "Espero."
The same could be said of other wishes given in this form: "Good night; sleep tight," or "Be safe/careful."
I have heard the entire wish given in both languages when someone is trying to emphasize their wish or to make it sound more sincere.
"I hope you have a nice day."
"Espero que pases un día muy lindo."
I have also heard both of these used sarcastically, where the opposite was actually meant.