Best time to visit the Greek Islands
Just about everyone agrees that the best time to visit Greece is spring and early summer (mid-Apr to mid-June) or autumn (Sept to mid-Oct). This way, you'll avoid the summer high season, with its inflated prices, hordes of tourists and high temperatures (heat waves of 100°F/+40C are routine).
In fact, unless you really like scorching heat, crowds, and overbooked planes, ferry boats, and hotels, August is to be avoided. In the spring, you'll see more wildflowers than you could have imagined -- and swim in a colder sea than you had hoped for!
In the autumn, you will enjoy golden days with a still-warm seas to swim in. One drawback: Off-season there are fewer boats and flights to the islands, where some shops, hotels, and restaurants do not open until June and then close in October.
Something to consider if you are coming to Greece in the spring: During Easter week, nearly every hotel room outside of Athens is booked well in advance by city Greeks who head to the country to celebrate Greece's most important holiday. Many sites and museums are closed Good Friday, Easter Saturday, and Easter Sunday, while many shops close on Good Friday and Easter Saturday. And when St. George's Day (usually celebrated Apr 23) falls in Lent, it is celebrated on the Monday after Easter Sunday, which just prolongs the Easter break. Also, the Feast of the Virgin on August 15 is an enormous holiday, especially on Tinos and Paros, but also on virtually every other Greek island and across the mainland.