What is Event, when Date is April 20, 2013?
CREATE TABLE "ufc_on_fox_s_nielsen_rating" (
  "event" text,
  "date" text,
  "rating" text,
  "share" text,
  "viewers" text
);
SELECT "event" FROM "ufc_on_fox_s_nielsen_rating" WHERE "date"='april 20, 2013';