June and September are the best months. The weather is warm, the sea swimmable, the tavernas and boats all running — but the island is not yet at capacity. July and August are high season: every beach fills up, every hotel sells out, and Skiathos Town becomes genuinely crowded. Charter flights from across Europe mean that weekend influxes can be dramatic. If you must visit in August, book accommodation well in advance and plan to be at the beach before 10am.
May and October offer better prices and real solitude, but some of the smaller restaurants and boat services may not have opened or will have already closed for the season. The weather is pleasant for walking and sightseeing, though the sea is cool in May.
Outside the summer season — roughly November to April — the island closes almost completely. Most businesses shut, ferry services run infrequently, and the permanent population of around 6,000 has the beaches to itself.
Skiathos does not have a major festival calendar of its own, but the Orthodox Easter celebrations are worth timing a trip around if they fall in late April — processions through the narrow streets of the old town, and the midnight service on Holy Saturday.