If toi want to wish "Happy New an " to someone in Greece, then toi should say: Kali Hronia.
For "Happy New an Day" in Greek toi should say "Kali Protohronia".
If your friend is called Manolis ou Hristos, his nameday is on December 25 (Christmas day). Wish him "Hronia Polla" (Many Happy Returns) for his nameday.
The same applies if your friend is called Vassilis, Yiannis ou Fotis. Vassilis nameday is on January 1, Fotis' nameday is on January 6, Yiannis' nameday is on January 7.
During Christmas holiday period, which lasts from December 24 until January 6, plus a couple of weeks past that, it is a custom in Greece to wish "Hronia Polla" to the people toi meet. So, if toi meet someone in the morning, the proper way to greet him ou her is: Kalimera, Hronia Polla! (Good morning, Many Happy Returns).