If FD starts in the middle of the rez, then you'll have to split your stay - And might be issues with tickets as the new rez will require a one day base ticket.
If FD ends in the middle of the rez, no problem. The offer is good until the last day for check-ins and good for up to 14 days. So, if the offer is good until 9/29 for example, you can actually check in on 9/29 and have FD for another two weeks.