How do you say Thank you very much have a wonderful day in French?

"Thank you very much! Have a wonderful day!" is the type of sentence that might make a French person giggle because it sounds so American. Americans are known for being excessively and insincerely polite. And no matter how you judge it, it's true that American social culture places more emphasis on smiling and emphatic expressions of good will toward strangers. So there really is no French translation of "have a wonderful day" because any translation that conveyed the literal meaning would sound silly. So you'd better just stick to

Merci beaucoup. Bonne journée. -- "mehr-see boh-koo bohn zhoor-nay"