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