With the 4-day Greek Islands pass, you have 4 travel days to use domestic ferries of the Attica group (including Blue Star ferries). After you've used these 4 travel days, it's 30% discount.

You can start wherever you like. Do note you have to book the ferries in chronological order, see the link above.

By the way, the best place to buy this pass in Germany is at the DB ticket office. It's only available on paper and at the ticket office, you don't pay any postage fee. And you don't have to wait for delivery.

Good afternoon,

Can I have 30% discount on ferries booked between the 4 travel days? 
For example:

day 1: travel day

day 2: I use 30% discount 

day 3: travel day

day 4: travel day

day 5: I use 30% discount

day 6: travel day

day 7: I use 30% discount

thank you very much for your answer

Sergi

Should be possible. In the FAQ about the Greek Islands pass, I read:

I want to make more than 4 domestic ferry trips in Greece. Is there a discount?

While using the Greek Islands Pass, you are entitled to buy separate tickets with a discount of 30% on all additional trips. Making use of this discount does not require the use of a travel day and can be done throughout the entire validity of the Pass. There is no limit on the number of times the discount can be used while your Pass is still valid.

Thank you very much for your answer.

Yesterday I activated two days for the Greek Islands Pass (4 days) and I booked some ferries for the two days, and today I tried to book a ferry with the 30% discount for one day after the two days activated and before to activate the last two. When I try to book this ferry with the 30%, Blue Star Ferries webpage says “The document number is not valid”… I don’t know what can I do :(

Activating the pass is not necessary.

What document number did you enter? It should be a pass cover number, which consists of a capital i and 8 digits, e.g. I12345678. If you have a mobile pass, you can generate one from your pass number:

Yes! It works!

but yesterday when I booked the ferries included in the day activated, I put the mobile pass number and worked too… maybe is because is different a 0 eur ticket than a 30% discount ticket…