For series number 256, what was the original air date?
CREATE TABLE "table1_17356106_1" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text
);
SELECT "original_air_date" FROM "table1_17356106_1" WHERE "series_num"=256;