Answer:
Monday 20th
Step-by-step explanation:
The FINRA rule says that an order to sell cannot be received unless the firm is sure they can deliver the securities within 2 business days.
The two business days here are "Friday 17th and Monday 20th".Saturday 18th and Sunday 19th are non business days.
It must also be noted that the order ticket to sell must carry the location of the order.