As soon as possible! It varies by place, but most hotels, inns, and Airbnb's will begin taking reservations by Nov/Dec 2024. House rentals tend to open up in January.
Downtown is very walkable. Bikes and mopeds are common to rent. There are car rentals available though they are fairly expensive. Uber and Lyft services are not available on the island. Instead, taxis are stationed around downtown areas and can be called for pick up. Note: taxis are cash only.
The island is manageable without a car, but if you are considering bringing one over, Point Judith, RI is the only ferry offering car service. Reservations are fairly expensive and competitive so be sure to book in early 2025. There are parking lots near the ferry terminals.
Block Island temperatures can range from mid 50s to high 70s in early June. Mornings can be foggy and evenings can get a bit cooler. The wedding will be outdoors with a tented reception that has sides to pull down in the event of inclement weather.