June to September is the main season. July and August are hot and dry, with water temperatures in the upper 20s. The island gets busy but never overwhelmed — it has no large hotels and the ferry journey keeps casual crowds away.
The carnival period (February or early March) is the most unusual time to visit. The Goat Dance is performed only during Apokries (the Greek pre-Lenten carnival), and Skyros Town fills with locals and visitors who have come specifically to see it. The weather will be cool and possibly wet, but the atmosphere is extraordinary.
May, June, and September offer the best balance: warm enough to swim, the island green from winter rains, and tavernas open without the summer crowds.
Avoid the ferry from Kymi on summer weekends — it books out fast. The flight from Athens is a more reliable option in peak season.