Greece is essentially a year-round destination, but the experience varies enormously by season.
Spring (April–June)
The best time to visit for most purposes. The landscape is green and wildflower-covered, temperatures are warm but not punishing (20–28°C), and the main archaeological sites can be explored in comfort. The sea is still cold for swimming in April, perfectly warm by June. Crowds are manageable everywhere except during Easter week, when the whole country moves. May is the sweet spot: long days, low prices, and the islands before the summer machine cranks up.
Summer (July–August)
The islands are in full swing and the sea is at its warmest, but July and August bring intense heat (regularly above 35°C on the mainland, plus the meltemi wind in the Aegean), sky-high prices, and heavy tourist traffic. Santorini and Mykonos in August are extraordinary in the sense that they are extraordinarily crowded and expensive. If you are set on summer, look at less-visited islands — Samos, Lesbos, Ikaria, Siros — or base yourself on the mainland. Athens in August is half-empty because locals flee to the coast — which means the city is actually quite pleasant.
Autumn (September–October)
September is arguably the ideal month. The sea is still warm (often warmer than in June), prices drop sharply after mid-August, the light softens into something extraordinary, and the crowds thin considerably. Harvest season brings local festivals across the country. October is cooler and perfect for hiking, driving the Peloponnese, or visiting Athens and Thessaloniki. Rain starts becoming likely in late October.
Winter (November–March)
Most island infrastructure shuts down from November to March — hotels close, ferries reduce to skeleton schedules, and many restaurants board their windows. The mainland, however, is a different story. Athens is a genuinely good winter city: museums are uncrowded, there are almost no tourists at the Acropolis, and the city's cultural life is at its fullest. Thessaloniki in winter has a lively café culture and excellent food. The mountain regions — Meteora, the Pindus range, the Pelion peninsula — are beautiful in winter, and Greek ski resorts (notably Parnassos) attract domestic visitors.
The Meltemi Wind
Worth knowing: the meltemi is a strong, dry northerly wind that blows across the Aegean from roughly July to mid-September. On exposed islands, it can keep temperatures bearable but also makes small boats unpleasant and can delay ferries. It blows hardest around Mykonos, Paros, and Naxos. Sheltered islands like Corfu in the west are unaffected.