Immediately. While we are scheduled ahead of the full tourist season that starts June 1st, May is still very popular and rooms/airfare are priced by availability. The longer you wait the higher the price.
Athens. From Athens you can book a flight through Aegean Air or Ryan Air for flights to Santorini. Most flights are around $70. You can also take the high speed ferry for $55. Our wedding planner said it is a nice ferry ride and you do get to see quite a bit while on it so recommends that you try it if you have 5 hours to spend.
Santorini has many different hotels/rentals that can accomodate different budgets. While we have listed a couple hotels we will be staying in, please find a place that fits your vacation. We will have transportation going from Fria to the venue and back but there are plenty of taxis on the island to get you where you need to go. I recommend looking at the venue Le Ciel on a map and seeing the different places that are available around it.
Right here! https://docs.google.com/document/d/1bvx6EFVqJaf-gQ0U4mEA7eSasX__itKxmdIMPjWs2j4/edit?usp=sharing