What was the airdate of 21 series number?
CREATE TABLE "season_1_2007" (
  "series_no" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" real
);
SELECT "original_air_date" FROM "season_1_2007" WHERE "series_no"=21;