Dies natalis means simply "birthday": dies is "day", and natalis is "natal; pertaining to birth" (natalis can be used by itself to mean "birthday" as well).
If you wanted to say "Happy Birthday" you would say Felix Dies Natalis, or simply Felix Natalis.
Was parliament mentioned in the Declaration of Independence
What did Thomas Jefferson mean when he inserted states into the Declaration of Independence
Read these words from the Declaration of Independence. What is the main purpose of this section of the document He has refused his Assent to Laws the most wholesome and necessary for the public good.