What was the date of the game with a decision of Lehtonen and attended by more than 17,731 people?
CREATE TABLE "december" (
  "date" text,
  "visitor" text,
  "score" text,
  "home" text,
  "decision" text,
  "attendance" real,
  "record" text
);
SELECT "date" FROM "december" WHERE "decision"='lehtonen' AND "attendance">'17,731';