Cornwall does not have the famousest (sic!) festivals in Britain. Glastonbury is not in Cornwall. Bestival, Download and the like may have cornered the market for evocative names but they, too, fall short in that one important respect. Not in Cornwall.
Our premier festival, Boardmasters, does appear in most festival-goers bucket list. It is held in Newquay, which is the surf capital of the Duchy of Cornwall.