The Seto Inland Sea is one of the driest, sunniest parts of Japan, and Takamatsu's climate is correspondingly mild. April for the cherry blossom in Ritsurin and Tamamo-koen; late May to June for the irises; November for the maple colours at Ritsurin, which are among the best in western Japan.
Summers are hot and humid but rarely as oppressive as Tokyo; winters are mild and almost free of snow. The Setouchi Triennale, held in roughly six-month windows across three years (spring, summer, autumn of the triennale year), is the busiest time, with hotel rooms in town becoming hard to get; check the dates if your trip coincides.