RE: Hawaii: Which island and which period?
B)NO question, Maui is by far the best for windsurf and the most windy. This is were all the Pro goes for training. You will be able to start practicing at Kanaha Beach Park (super safe to learn and make progress), and depending on your progress try Camp One and Spreckelsville and may be for the last month Hookipa.
Best to find a accomodation on the Northshore: in Paia or Haiku.
Best time of the year: between may and october.
You will find information on the link below: