When did the episode air that had 7.3 viewers?
CREATE TABLE "season_1_2008_2009_weekly_ratings" (
  "episode" text,
  "air_date" text,
  "timeslot" text,
  "18_49" text,
  "viewers" real
);
SELECT "air_date" FROM "season_1_2008_2009_weekly_ratings" WHERE "viewers"=7.3;