What date was the show's weekly ranking 12?
CREATE TABLE "series_7_2013" (
  "episode" text,
  "date" text,
  "official_itv_rating_millions" text,
  "weekly_rank" real,
  "share_pct" text,
  "official_itv_hd_rating_millions" text,
  "total_itv_viewers_millions" text
);
SELECT "date" FROM "series_7_2013" WHERE "weekly_rank"=12;