Somerville will hold Riverfest on Saturday, a daylong celebration along the Mystic River at the waterfront park near Assembly Square. Starting at noon, there will be live musical performances and a children’s area with structured entertainment. A half-dozen food trucks will be on hand to satisfy appetites. For children, there will be face painting, balloon twisting, and games. Teens and adults can try their skill on a rock-climbing wall, or enjoy perusing a show of Edsel cars, the brand that once rolled from a Ford plant on the site of the future Assembly Square development.