Spring (April to June) and autumn (September to October) are the best times to visit Ayvalık. The weather is warm and clear, the light across the bay is at its most beautiful, and the town hasn't yet been swamped by the summer holiday crowd. Olive harvest runs from late October through December and is a good time to see the groves in action and buy fresh-pressed oil.
July and August are hot, humid, and busy. Turkish domestic tourism peaks sharply in these months: accommodation prices spike, the waterfront fills with day-trippers from İzmir and beyond, and Cunda's restaurants can get overloaded. If you visit in summer, arrive early in the day and plan to move slowly.
Winter is quiet to the point of being sleepy. Some restaurants and guesthouses on Cunda close entirely. The town itself stays open for locals, and the old quarter is atmospheric without the crowds — but ferry connections to Lesbos are suspended and the archipelago boats stop running.