Formal:
* Bore da, cael di ddiwrnod da. (This is the most literal translation, but it is a bit stiff)
* Bore da, gobeithio bod di'n cael diwrnod da. (This is more natural, meaning "Good morning, I hope you have a good day.")
Informal:
* Bore da, dymuno di ddiwrnod da i ti! (This is a friendly way to say it, meaning "Good morning, I wish you a good day!")
* Bore da, caw di ddiwrnod braf! (This is a more casual way to say "Good morning, have a nice day!")
You can also use "Bore da!" or "Diwrnod da i ti!" on their own to mean "Good morning!" or "Have a good day!"
The best way to say it depends on the context and your relationship with the person you are talking to.