Book through either Catalina Express or the Catalina Flyer websites. Make sure your ferry is going to and from AVALON and not Two Harbors. Booking for May 2019 opens up in March 2019. https://www.catalinaexpress.com/ http://www.catalinainfo.com/
Once you get to Avalon you will realize that the town is quite small and most people drive golf carts or walk. There are taxis available when going further distances. If you book at Hamilton Cove, many of the condos come with a golf cart to use during your stay.
In general the weather in Avalon in May can be variable. We suggest watching the forecast as the date approaches. As for the wedding events we ask people to dress semi-formal for the ceremony and reception.