We're often asked “When is the best time to go on vacation in Greece?", and the answer is often complex. It will depend on many things including your interests, exactly where you want to visit and why you're travelling. One person's best time can be another's worst!
However, often a concern about the weather underlies the question of 'the best time to go'.
The vacation season for most of the Greek islands starts in April (spring season) and end in late October (autumn season).
The peak holiday season in Greece is from early July to the end of August, when temperatures are higher, and crowds of foreigners and locals alike travel to the Greek islands. You won't miss out on warm weather if you come in June or September , excellent times almost everywhere but particularly in the islands. An exception to this pattern, however, is the north-mainland coast - notably the Halkidiki peninsula - and the islands of Samothrαki and Thαssos, which only really cater to visitors during July and August . In October, for most of that month the "summer of Αgios Dimitrios" prevails, and the Aegean islands, the southerly Dodecanese and Crete are extremely pleasant. Autumn in general is beautiful; the light is softer, the sea often balmier than the air, and the colors subtler.
December to March are the coldest and least reliable months, though even then there are many crystal-clear, fine days, and the glorious lowland flowers begin to bloom very early in spring. The more northerly latitudes and high altitudes of course endure far colder and wetter conditions, with the mountains themselves under snow from November to May. The mildest winter climate is to be found on Rhodes, or in the southeastern parts of Crete. As spring slowly warms up, April is still uncertain, though superb for wild flowers, green landscapes and photography; by May the weather is more generally predictable, and Crete, the Peloponnese, the Ionian islands and the Cyclades are perhaps at their best, even if the sea is still a little cool for swimming. Note, however, that these are the historical patterns as observed until the early 1990s; thanks to global warming, recent years have seen erratic climate, with unusually cold Mays, warm Octobers, little (and late) rain, plus very early spring flowering.
Other factors that affect the timing of your Greek travels have to do with the level of tourism and the amenities provided. Service standards, particularly in tavernas, slip under peak-season pressure, and room rates top out from July to September (as well as during Easter or Christmas week). If you can only visit during midsummer, reserve a package well in advance, or plan your itinerary off the beaten track: you might for example explore the less obvious parts of the Peloponnese or the northern mainland, or island-hop with an eye for the remoter places.
Out of season , especially between late October and late April, you have to contend with reduced ferry services to the islands (and nonexistent hydrofoils or catamarans), plus fairly skeletal facilities when you arrive. You will, however, find reasonable service on all main routes and at least one hotel and taverna open in the port or main town of all but the tiniest isles. On the mainland, winter travel poses no special difficulties except, of course, in mountain villages either cut off by snow or (at weekends especially) monopolized by avid Greek skiers.