Spring (April–May) is excellent. The island is green, the wildflowers are out, and the birdwatching is at its peak — Lesbos lies on a major migration flyway and draws twitchers from across Europe, particularly for the spring passage of raptors and waders. The days are warm enough for cafe-sitting and sightseeing without the summer heat.
Summer (June–August) is hot, crowded, and expensive by Greek island standards. Temperatures regularly reach 35°C and the ferry traffic from Athens means Mytilini fills up in July and August. The compensations are a lively outdoor scene and long warm evenings. The Lesbos International Documentary Festival takes place in late July.
Autumn (September–October) rivals spring. The sea stays warm for swimming well into October, the olive harvest begins in October, crowds thin dramatically after mid-September, and prices drop. The landscape turns golden and the light is softer.
Winter (November–March) is mild by northern European standards but damp. The island gets proper rain in winter. Most tourist infrastructure closes, but the city itself carries on — the cafes fill up, the ferries run, and you'll experience Mytilini as a real Aegean town rather than a visitor destination.