Fine weather is due to accompany travelers over this long weekend.
Forecasters expect to temperatures to fluctuate between 26 and 31 Celsius (79-88F).
Most ferries departing Athens on Thursday, Friday and Saturday reported being fully booked.
So far in August, 363,794 passengers have sailed from Piraeus.
The port authority can be contacted by calling 14541 24 hours a day for information about arrivals and departures from Piraeus.
Passengers taking their cars are advised to arrive 90 minutes before their vessel is due to sail.
Details about sailings are also available by dialling 14944.