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