Greece cruises are available year round, though the most popular time to cruise is from May through October. Often itineraries include stops in other ports in the Mediterranean like Istanbul or Venice. Most trips are between seven and 28 days in length and call at ports on the mainland and in the Greek isles.
The heaviest tourist traffic in the region coincides with the high temperatures of the summer months. The hottest month is August, but even then there's often a cool breeze to alleviate the summer heat. Travel during the shoulder seasons in the spring (April and May), fall (September and October) or winter (November to March) makes for a cooler and calmer trip. An added bonus for spring is the draw of blooming wildflowers. The temperatures are still comfortable during these months -- the high temperature in Athens is around 70 degrees in April and October -- but the number of people crowding the historic sites and packing the beaches will be greatly reduced.