The cheapest time of the year, according to airlines, to fly to Greece is in December. I am quite sure this would be the same for most places. This is the farthest from the peak season so it would make sense to be cheaper. Now if you want to holiday in Greece for the hot weather and to see all the tourists then this wouldn’t work for you. If this is not an issue then you can more than likely save up to 50% flying in December.