What is the Date at memorial stadium • minneapolis, mn with an Attendance of 20,000, and a Result of w52-0?
CREATE TABLE "schedule" (
  "date" text,
  "opponentnum" text,
  "site" text,
  "result" text,
  "attendance" text
);
SELECT "date" FROM "schedule" WHERE "site"='memorial stadium • minneapolis, mn' AND "attendance"='20,000' AND "result"='w52-0';