Middlebury’s exciting Festival on-the-Green continues on Tuesday. Here are the highlights. At noon, Stephen Gratto will present a Brown Bag Special performance for the whole family. At 7 p.m., the Miles Donahue Quintet, described as “a jazz treasure,” hits the stage. Then, at 8:30 p.m., Darlingside will perform. “A quartet with a rich line in acoustic textures and chamber-rock dynamics,” says one enthusiastic reviewer.