Question: when was the customer with the most order's last order placed?
SQL: select date_order_placed from Orders where customer_id in  ( SELECT  T1.customer_id  FROM Orders AS T1 JOIN Customers AS T2 ON T1.customer_id   =   T2.customer_id GROUP BY T1.customer_id ORDER BY count ( * )  DESC LIMIT 1 )  order by date_order_placed desc limit 1