CREATE TABLE table_11991 ("Race" TEXT, "Circuit" TEXT, "Date" TEXT, "Pole position" TEXT, "Fastest lap" TEXT, "Winning driver" TEXT, "Constructor" TEXT, "Tyre" TEXT, "Report" TEXT)

When was the swiss grand prix?
SELECT "Date" FROM table_11991 WHERE "Race" = 'swiss grand prix'
SELECT "Date" FROM "table_11991" WHERE "Race" = 'swiss grand prix'