Currently, a copy of your active license will suffice.
There are no black-out dates for this discount, but, if the hotel is at 80% occupancy, all discount offers are withdrawn. BUt, if you want the Dolphin and it is 80% reserved, and the Swan (my favorite!) is less than 80, you can stay at the Swan or vice cersa.
Also, if you can only reserve some of your dates, keep checking back on Friday mornings, as that is when new discount-available dates are released into the computer.