There are a lot of ceilidhs in PEI over the summer. As a matter of fact you would be hard pressed to get to all of them. However, there is a great website (www.ceilidhs.ca/pei-ceilidhs.html) that has a very robust calendar listing all of them.
I highly recommend the Weekly Music Sessions at the Old Triangle in Charlottetown at 2:00 pm on Sundays. Roy is a great musician and will have some high-quality guests joining him throughout the summer.