We had a huge disspointment yesterday when long awaited friends visiting from USA, were denied their flight in USA, because they had valid passports, valid though only for the next 80 days.
In order to enter Schengen agreement countries ( more than 20 countries in Europe, including France, Germany and Greece for example ) your passport has to be valid FOR MORE than three months than your intended stay.
American visitors are not in need of a visa to enter Greece, still this rule applies. So, check you passports.
More info, regarding US passport holders, in US Department of State info page at the following link/ Please do take a look.