Named Washington’s “Artist of the Year” in 2003, this roots-rock band — part rock, part blues, part bluegrass — has since relocated to Nashville, but its roots are still here, so any local performance promises to be sweet. And how could it get any sweeter than free? They return for what’s become an annual concert on the Millennium Stage. Tuesday, Dec. 20, at 6 p.m. Kennedy Center Millennium Stage. Tickets are free. Call 202-467-4600 or visit kennedy-center.org.