If you want to wish "Merry Christmas" to someone in Greece, then you should say: Kala Hristouyienna.
If you want to wish "Happy New Year " to someone in Greece, then you should say: Kali Hronia.
For "Happy New Year Day" in Greek you should say "Kali Protohronia".
If your friend is called Manolis or 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 or 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 you meet. So, if you meet someone in the morning, the proper way to greet him or her is: Kalimera, Hronia Polla! (Good morning, Many Happy Returns).