July and August are the peak — long warm days, all the shops and restaurants open, every ferry full, every restaurant booked, every beach crowded. You see the island at its busiest but also at its most alive.
Late May through June and September through early October are the better travel windows for most visitors. The weather is still good (June can be cool and damp; September is often the warmest month of the year), the ferries are bookable, restaurants are reservable, and the island feels much closer to its quieter year-round self.
Winter is bleak and beautiful in equal measure. Most restaurants and many shops shut down between November and April. Some travellers love it — empty beaches, dramatic light, the small year-round community easier to meet — but expect limited services. The Christmas in Edgartown and Vineyard Artisans Festival weekends bring brief flurries of off-season activity.