This festival of art, music, drama and dance comes around every three years and has a packed calendar of events occurring on multiple Inland Sea islands, many on Naoshima and Teshima. Previous schedules have been spread across three seasons (spring, summer and autumn.)
Check the excellent website for the lowdown on events and ferry passes. The recommended three-season festival 'passport' ticket (¥4000 if bought in advance, or ¥5000 on site), which covers entry to most sites, offers significant savings for those visiting a number of sites and museums. Book your accommodation well ahead if you plan on staying on Naoshima or even in Takamatsu during the festival.