St. Constantinus and St. Helen.
Greeks have 'name days' besides birthdays as nearly each day of the year is dedicated to one of the Christian saints.
So, the day you are born means: you have a birthday and a name day related to that Saint of that particular day you are born on.
Although birthdays became more common, name days are still celebrated.
Yes, the Greek Orthodox religion plays a significant part in the lives of the Orthodox Greeks.
Click here for a list of name days. Or this more specific list.