GREY'S ANATOMY - "Six Days" - After a successful operation on his heart, George's father undergoes surgery for his cancer, Thatcher Grey visits Seattle Grace in order to see his new granddaughter, and Meredith discovers that Derek has trouble sleeping soundly, on "Grey's Anatomy," THURSDAY, JANUARY 4 (9:00-10:01 p.m., ET) on the ABC Television Network. (ABC/MICHAEL DESMOND) CHANDRA WILSON, KATHERINE HEIGL