You don't have to play for 40 years to fashion a certain authenticity, but it doesn't hurt. Robin and Linda Williams offer everything you could ask of American folk music—from sweet harmonies to crisp, lively playing, plus well-crafted songs whose concept of rustic living honors long-standing traditions rather than a Green Acres conception of the country. As their appearances on Prairie Home Companion suggest, they aren't beyond wit or romanticism, but their reputation's built on songs like the slyly poignant "Men With Guns," their swinging "Train Whistle Blues" or haunting "Don't Let Me Come Home a Stranger." Free/ 6 p.m. —Chris Parker