GREY'S ANATOMY - "What a Difference a Day Makes" - Bailey performs a test on Izzie as the imagined spectre of Denny Duquette watches ominously, on "Grey's Anatomy," THURSDAY, MAY 7 (9:00-10:02 p.m., ET) on the ABC Television Network. (ABC/SCOTT GARFIELD) KATHERINE HEIGL, CHANDRA WILSON, JEFFREY DEAN MORGAN