Which date was the Memorial Tournament held on, when Payne Stewart was runner-up?
CREATE TABLE "pga_tour_wins_20" (
  "date" text,
  "tournament" text,
  "winning_score" text,
  "margin_of_victory" text,
  "runner_s_up" text
);
SELECT "date" FROM "pga_tour_wins_20" WHERE "tournament"='memorial tournament' AND "runner_s_up"='payne stewart';