Formal:
* Magkaroon ka ng magandang araw. (This is the most literal translation.)
Informal:
* Magandang araw sa iyo! (Good day to you!)
* Enjoy your day! (This is often used in Filipino, especially in a casual setting.)
* Sana maganda ang araw mo! (I hope you have a good day!)
The best way to choose which translation to use depends on the context and your relationship with the person you're speaking to.