Siracusa is a very touristic city and May is considered high-season. Siracusa downtown is a little island called ORTIGIA, connected with the rest of the city by two bridges. Ortigia is small and everything is at walkable distance, therefore there is no need for renting a car. The congress will be held in Ortigia, and it would be handy to book a room there. As Ortigia is small, there is limited availability of hotels but a great offer of holiday apartments and b&b's.
Moreover, May is the time of the year where the Greek Tragedies at the ancient amphitheater are performed. Tourists from all over the world come to watch them! If you are considering participating to the congress, it would be wise to book a room far in advance.