The summer months (from December to February) are when Waiheke is most lively, with music events most weekends. This is also when the island is at its busiest, so you’ll need to book in advance for lunch reservations or desirable accommodation. You’ll find more things open on weekends – some galleries open only on Saturdays and Sundays. Earlier in the week, smaller cellar doors and restaurants are likely to be closed.
If you’re just going for a day, aim to be there for lunch time or earlier. It’s easier to find places to eat before 3pm, when many cafes and restaurants close.