CREATE TABLE table_21906 ("Week" FLOAT, "Date" CLOB, "Opponent" CLOB, "Result" CLOB, "Game site" CLOB, "Record" CLOB, "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'