As DisBuckMan said, your check-in date has to be during the FD dates. If FD starts in the middle of your stay, you can do a split stay and get FD for the last days of your trip, but that does get a little complicated since the tickets have to be attached to the FD part of the stay, and you'll need to activate them early. Then there is the possibility of having to move rooms or even resorts to get the FD.