WTOP Entertainment Reporter Jason Fraley wishes David Letterman a happy 75th birthday with this flashback episode to his 2017 Mark Twain Prize for American Humor at the Kennedy Center. You'll also hear from Letterman's longtime announcer Alan Kalter and comedy colleagues like Jimmy Kimmel, John Mulaney, Jimmie Walker, Martin Short, Al Franken and Bruce Vilanch. (Theme Music: Scott Buckley's "Clarion")