If someone's having a birthday, you want to say to them things like 'Have a good day!', and 'Happy birthday!'. For example, if you were writing a card, you might say something like:
'Dear (their name),
Happy birthday! I hope you have a good day, many best wishes for the future!
[Love] from (your name) [xo].'
The 'love' and 'xo' are optional, depending on if you're friends with the person or not.
I hope this helps in any way :)