On what date was there a game in which the opponent was the Detroit Tigers, and the attendance was 38,639?
CREATE TABLE "game_log" (
  "date" text,
  "opponent" text,
  "score" text,
  "loss" text,
  "attendance" text,
  "record" text
);
SELECT "date" FROM "game_log" WHERE "opponent"='detroit tigers' AND "attendance"='38,639';