Philadelphia is now hosting three major art-world happenings—and that’s not including the spectator sport of the board-vs.-director battle at the Philadelphia Art Museum. But that institution is where Dreamworld, the 100th anniversary show of surrealism that has been touring Europe, is making its only American stop, which makes it a major event. Elsewhere in the city, the Barnes Foundation is presenting Henri Rousseau: A Painter’s Secrets, which highlights the institution’s large holdings of the painter’s work and brings many important examples together for the first time. Then there’s the recently opened Calder Gardens, which is a tribute to one of the city’s native sons, Alexander Calder.