Bang on a Can's 12-hour run of music this weekend at the World Financial Center was wondrous for the sight of hundreds sitting on the steps of what is essentially a mall atrium, yet acting completely city-like, with boyfriends and babies and books in tow, making ad hoc picnics and listening to exquisite sounds, hour after hour. A highlight was the Brooklyn Youth Chorus's performance. Tip for next year: it's worth getting there early to see the sun set behind the players.