His father gave Peter a magic bean. He assured him that if he planted this bean, a huge bean stalk would grow immediately. On Tuesday night, Peter planted it and placed it on his windowsill in his room. When Peter looked at the pot the next morning, he saw a small plant just beginning to sprout from the soil. The next morning, the plant had doubled in size, and from then on, each morning he observed that the plant was doubling. By the following Friday (that is, one week later), the plant had filled the entire room. On which day did the magical plant cover half of the room?