Formal:
* Tenha um dia fabuloso! (This is the most direct translation and is appropriate for formal situations.)
Informal:
* Tenha um dia ótimo! (This means "have a great day" and is more common in casual settings.)
* Tenha um dia incrível! (This means "have an amazing day" and is even more informal.)
More expressive:
* Que o seu dia seja fabuloso! (This literally means "May your day be fabulous!")
* Desejo um dia fantástico! (This means "I wish you a fantastic day!")
Other options:
* Aproveite o seu dia! (This means "Enjoy your day!" and is a good option if you want to be more concise.)
The best way to say "have a fabulous day" in Portuguese depends on the specific situation and the person you are talking to. Consider the level of formality and your relationship with the person when choosing the right phrase.