Well, it depends on what you exactly mean : -> if you just want to say that the day is nice : "What a nice day !" => "Quelle belle journée !" -> if you want to wish someone, when leaving, a nice day : "Have a nice day !" => "Bonne journeé !" I hope it helps.

It means :" Amusez-vous bien"

we say : "belle journée "