Just about everyone agrees that the best time to visit is 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.
I've been visiting Greece since the 1960s, and for the last decade I've spent about half the year in Greece, most of it in a village in the Peloponnese, south of Athens. The one thing I can swear to is that the weather is getting less predictable every year! When Greeks talk about the weather, increasingly they say that everything is ano kato (upside down). Villages where snow was almost unknown had blizzards in 2010/11. Areas where February always meant steady rain, saw hardly a drop all month. One thing everyone agrees on: the winters are colder (sometimes drier, sometimes wetter, sometimes with unusual bursts of warm weather). Summers are just plain hotter (and usually drier) than even 10 years ago. Even though our temperature chart for Athens reflects some sound statistics (this is the average daily temperature, not the daytime high), don't be surprised if you find deviations from it when you visit Greece.
Every day in Greece is sacred to one or more saints. That means that every day, at least one saint (and everyone named for that saint) is being celebrated. Tiny chapels that are used only once a year are opened for a church service followed by all-day wining and dining. If you're lucky, you'll stumble on one of these celebrations.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.