What date did the Chicago Bears beat the Green bay Packers 31-20?
CREATE TABLE "1950s_bears_14_5_1" (
  "year" real,
  "date" text,
  "winner" text,
  "result" text,
  "loser" text,
  "attendance" real,
  "location" text
);
SELECT "date" FROM "1950s_bears_14_5_1" WHERE "loser"='green bay packers' AND "location"='green bay';