When was the marriage of the person who died on 17 December 1471?
CREATE TABLE "1405_1482" (
  "name" text,
  "father" text,
  "birth" text,
  "marriage" text,
  "became_countess" text,
  "ceased_to_be_countess" text,
  "death" text,
  "spouse" text
);
SELECT "marriage" FROM "1405_1482" WHERE "death"='17 december 1471';