The peak season is May through October, when the boardwalk is busy, the river cruises and whale-watch boats are running out of the harbour, and the Plum Island beach is open. September and early October are the best weeks: warm days, cool nights, autumn colour in the trees lining High Street, and the September hawk migration over Parker River.
Winter is genuinely cold — the river ices up against the docks in January — but the town stays open, the restaurants thin out pleasantly, and the snowy owls arrive on Plum Island. The Yankee Homecoming festival in early August is the city's big week; expect crowds and book restaurants ahead.