I would like to know how to say Happy Saints Patrick’s Day. I think happy is “Felicem” and saints is “Sancti”
You could literally translate that as Felicem diem Sancti Patricii.
The literal translation given by Rhodopeius is fine, but it might be more natural in Latin to use the word festum “holiday/festival” instead of just “day”. That would make it Felix festum Sancti Patricii.