What was the main date of the round with 20 fixtures?
CREATE TABLE "calendar" (
  "round" text,
  "main_date" text,
  "number_of_fixtures" real,
  "clubs" text,
  "new_entries_this_round" text,
  "prize_money" text
);
SELECT "main_date" FROM "calendar" WHERE "number_of_fixtures"=20;