What is the original air date when there were 12.81 million u.s viewers?
CREATE TABLE "table1_11404452_1" (
  "series_num" real,
  "episode_title" text,
  "writer_s" text,
  "director" text,
  "u_s_viewers_millions" text,
  "original_air_date" text
);
SELECT "original_air_date" FROM "table1_11404452_1" WHERE "u_s_viewers_millions"='12.81';