When did the season finale ranked at 73 first air?
CREATE TABLE "nielsen_ratings" (
  "season" text,
  "timeslot_times_listed_are_eastern_time" text,
  "season_premiere" text,
  "season_finale" text,
  "tv_season" text,
  "ranking" real,
  "viewers_in_millions_of_households" text
);
SELECT "season_finale" FROM "nielsen_ratings" WHERE "ranking"=73;