Passe une belle journée !
OR Passe une bonne journée !
"Have a lovely day!" would be "Passe une belle journée!" in french. Hope this helps :)
In French, "have a lovely day" can be translated as either "Passe une belle journée !" or "Passe une bonne journée !" The first option conveys a warmer sentiment, while the second is more neutral. Both are appropriate for wishing someone a pleasant day.
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