Complex sentence: Its most characteristic cultural event is the Interceltic Festival of Avilés, which takes place on summer, with people coming from Brittany, Ireland, Wales, Scotland, Galicia and Asturias itself.
Split into simpler sentences: Its most characteristic cultural event is the Interceltic Festival of Avilés. That occurs in summer, with people coming from Brittany, Ireland, Wales, Scotland, Galicia and Asturias itself.