May, June, and September are the best months. Temperatures are warm (22–28°C), the sea is swimmable, accommodation is available without booking months in advance, and the island is busy but not overwhelmed. July and August bring serious heat (up to 35°C), cruise-ship crowds that turn Oia into a traffic jam, and hotel prices at their peak. The caldera cliff walk is miserable in full summer heat.
April and October are quieter and cheaper, with milder weather and fewer visitors. Some smaller restaurants and accommodation close from November to March, and ferry schedules thin out considerably. Santorini in winter is a very different island — tranquil and local, with the villages returned to their permanent residents.
The island has no major local festivals that drive visitor numbers, but the Orthodox Easter celebrations across Greece are worth timing a trip around if you can: Greek Easter is a genuinely spectacular affair, with candlelit midnight processions and bonfires on the eve.