"Have a good day!"
Translation:O zi bună!
„O zi bună”” is simply a shorter form of „să aveți o zi bună!”. „Aveți o zi bună” would mean ''you are having a good day" which makes no sense here.
„Să aveți” is the so-called conjunctive form of the verb and it has many uses in Romanian (it's worth learning it properly). One of its uses is in imperative-like sentences, like „să aveți o zi bună!”