CREATE TABLE table_11997 ("Date" TEXT, "Round" TEXT, "Attendance" TEXT, "Opposition" TEXT, "Stadium" TEXT)

When was the opposition sydney fc on round 19?
SELECT "Date" FROM table_11997 WHERE "Opposition" = 'sydney fc' AND "Round" = 'round 19'
SELECT "Date" FROM "table_11997" WHERE "Opposition" = 'sydney fc' AND "Round" = 'round 19'