June and September are the best months. The sea is warm, the island is not overrun, and the light in September is extraordinary. July and August bring the full weight of Greek summer — temperatures above 35°C, beaches packed with Greek and Bulgarian families, and roads choked around the ferry ports. The island is genuinely popular with visitors from Bulgaria and northern Greece who can reach it easily, which means midsummer is busier than you might expect for an island of this size.
Spring is quietly excellent. Wildflowers cover the hillsides from April, the beaches are empty, and the tavernas are just opening up for the season. The pine forests are at their most fragrant after winter rain. Birdwatchers will find the island productive on spring migration.
The ferry from Kavala (90 minutes) and the shorter crossing from Keramoti (35 minutes, just the car ferry) run year-round, though services thin out considerably from November to March. The island does not shut down entirely in winter — locals live here — but most tourist businesses close. If you want Thassos to yourself, go in October or early November: the sea is still swimmable, the crowds are gone, and the tavernas are in a generous end-of-season mood.