Each year, Blue Curtain and the Princeton Recreation Dept. present two concerts of music, usually from Africa or Latin America. The performers tend to be first rate, and the setting at Pettoranello Gardens is highly conducive. For more info, Blue Curtain has a facebook page, and there's a brief writeup on the two concerts here. Two bands play tonight, with the first one, some Brazilian choro, starting at 7pm.
The "blue" curtain concept becomes more clear in these photos.