CREATE TABLE table_21906 ("Week" FLOAT, "Date" TEXT, "Opponent" TEXT, "Result" TEXT, "Game site" TEXT, "Record" TEXT, "Attendance" FLOAT)

When did they play at the Cotton Bowl?
SELECT "Date" FROM table_21906 WHERE "Game site" = 'Cotton Bowl'
SELECT "Date" FROM "table_21906" WHERE "Game site" = 'Cotton Bowl'