Before the sail date , usually 3 - 4 weeks early. I believe NCL is Casinos at Sea.
You set a minimum of $5000.00 and provide the on board casino a signed check after boarding. Then you get a marker from the pitboss when needed. At end of the cruise you will go to cashier and fill out the amount you owe on the check or pay off the balance and get your check back.
Hope this helps