When were the callback auditions at Hilton Riverside Hotel held?
CREATE TABLE "regional_auditions" (
  "episode_air_date" text,
  "audition_city" text,
  "first_audition_date" text,
  "audition_venue" text,
  "callback_audition_date" text,
  "callback_venue" text,
  "golden_tickets" text
);
SELECT "callback_audition_date" FROM "regional_auditions" WHERE "callback_venue"='Hilton Riverside Hotel';