If 당신 want to wish "Happy New 년 " to someone in Greece, then 당신 should say: Kali Hronia.
For "Happy New 년 Day" in Greek 당신 should say "Kali Protohronia".
If your friend is called Manolis 또는 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 또는 Fotis. Vassilis nameday is on January 1, Fotis' nameday is on January 6, Yiannis' nameday is on January 7.
During 크리스마스 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 당신 meet. So, if 당신 meet someone in the morning, the proper way to greet him 또는 her is: Kalimera, Hronia Polla! (Good morning, Many Happy Returns).