What was the original air date for season number 2?
CREATE TABLE "table1_21781578_2" (
  "season_no" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text,
  "u_s_viewers_millions" text
);
SELECT "original_air_date" FROM "table1_21781578_2" WHERE "season_no"=2;