You can book as far as 11 months out. I don't know which airline you prefer but we fly Delta. I think Delta doesn't have direct flights to Athens from Atlanta. Coming back, you will be flying from Crete (airport in Heraklion) and you most likely will need to fly Aegean Air back to Athens or another major European city (if you decide to make a stop to another city). Aegean Air is not available for booking till about 6-7 months before. I booked that separately for our Greek Odyssey trip this year.