Tell me the date for acts of 6 bands and year larger than 1981 for monsters of rock
CREATE TABLE "1980s" (
  "year" real,
  "date" text,
  "event" text,
  "days" text,
  "stages" text,
  "acts" text
);
SELECT "date" FROM "1980s" WHERE "acts"='6 bands' AND "year">1981 AND "event"='monsters of rock';