A few years ago I was travelling back to Athens from Paros on a weekend in mid-September and all the ferries were full, including the big Blue Stars. I think it depends partly upon which Sunday you mean—if it is the 9th I would book ahead, but if you mean the 16th it would probably be okay to wait until a few days ahead. A lot of people are returning from the islands around that time, and weekends are the busiest.
I am never 100% certain of where I will be that much in advance, so pre-booking a long way ahead is simply not possible. I have genuinely not seen ferries absolutely full for my return trip, however. Although, I usually return to Athens on a Wednesday or Thursday.
I would never pre-book "a long way ahead", but usually buy my ferry tickets a day or two in advance, or on the day when first arriving in Greece. I was taken by surprise on that particular occasion, and managed to get a VIP ticket on a highspeed when Blue Star was full. Up until then I assumed ferries were never full in September, and that's usually, but not always, the case. Ferries returning to Athens on Sundays in the first half of September are busy.
It has to be Sunday 23September Mykonos to Piraeus as we have our Athens hotel booked. Do you think that time should be ok? We don't normally take a Sunday ferry by choice and just started to wonder if it would be an issue or not as it is a bit later in the season. Do travel agencies on the islands have any idea of passenger availability numbers? Think we will decide once we are in Greece if we can leave it for a bit, just don't want to get caught out. Thanks
No problem with the 23rd.
The fact that many ferrioes are full the 2nd weekend of September has probably to do with the fact that many families return after visits to family/relatives on the islands. They return at that time because school restarts for their children mid-September.
But returning the 23rd should be no problem if you buy your tickets 1-2 days before your trip back to the mainland.Edited: 06 August 2018, 14:52