CREATE TABLE table_54748 ("Father" TEXT, "Birth" TEXT, "Marriage" TEXT, "Became Queen" TEXT, "Ceased to be Queen" TEXT, "Death" TEXT, "Spouse" TEXT)

When did the person who died on 6 Dec 1240 get married?
SELECT "Marriage" FROM table_54748 WHERE "Death" = '6 dec 1240'
SELECT "Marriage" FROM "table_54748" WHERE "Death" = '6 dec 1240'