Himeji is good year-round, but the two famous windows are cherry blossom (late March to early April) and autumn colour (mid-November). Cherry season pulls heavy crowds around the castle — hanami at Himeji-jo is one of the great photographs of Japan — and weekends in early April are essentially elbow-to-elbow. The autumn maples in Koko-en are calmer and often more rewarding.
Summer is hot and humid; the castle has little shade and reflective white walls that make the climb sweaty work. The Himeji Castle Festival in August (Yukata Matsuri) is worth catching if you are in town then. Winter is mild, clear, and uncrowded — the white walls against a winter blue sky are arguably the castle at its best.