As we previously reported here is the official word from ABC.
"Lost" Finale Airs 9:00-11:00 p.m. on Thursday, May 29
ABC Entertainment announces special two-hour finales of "Grey's Anatomy" and "Lost." The "Grey's Anatomy" finale airs Thursday, May 22 (9:00-11:00 p.m., ET), and the "Lost" finale airs Thursday, May 29 (9:00-11:00 p.m., ET) on the ABC Television Network. This marks an additional hour for each show this season, for a total of 17 hours for "Grey's Anatomy" and 14 hours for "Lost." (*Editor's note: "Lost" will be pre-empted on May 22 due to the "Grey's Anatomy" finale.)
"Grey's Anatomy" returns with original episodes on Thursday, April 24 (9:00-10:00 p.m., ET), and "Lost" returns with originals that night as well, when it moves to its new 10:00 p.m. time slot.
Source: ABC