Summer (June–August). The obvious season — warm days, long evenings, the ferries running on full schedule, every restaurant patio open. Also the busiest: book ahead for dinner and accommodation. Average highs around 25°C.
Autumn (September–October). Quieter, often warmer than expected, with the inland foliage at its peak in early to mid-October. The single best time to come if you want Maine without the crowds. Lobster prices are usually at their lowest after Labor Day.
Winter (November–March). Cold, often snowy, with short days. The Old Port shrinks back to its locals, the ferry schedule thins out, and prices drop sharply. The PMA and the museums are pleasures on a grey day. White Mountain skiing is an easy day trip.
Spring (April–May). The least reliable season — mud season inland, mixed weather on the coast. May warms up quickly and the city begins to open back up. Mosquitoes start in late May and continue through June, particularly inland.