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".
How Christmas is celebrated in Greece and lots of other countries around the world.. In Greek Happy/Merry Christmas is 'Kala Christougenna'.. In the Greek Orthodox Church, Epiphany celebrates Jesus's baptism when he was a man.