Skiathos is a beautiful island no matter what time of the year you choose to travel. The holiday season generally starts the first Friday of may for charter flights from the UK. Flights are then easy to come by right up to October. The four months between June and September are the hottest months.
As the island does get hot and has a refreshing breeze, it is easy to end up getting burnt, please stay safe in Skiathos
In terms of rainfall, July is the driest month, but they really isn’t much rain during the holiday season