The FD BB check-in dates are based on when resort bookings may have been softer last year. My guess is that the end of May with Memorial Day, and then the first weeks of June, with the start of summer in FL and southern states, brings enough resort guests that they don't need to put out an offer then. It's also not as hot in early June as it is in July-September, which may detract some from coming. It's also just the beginning of hurricane season and not "peak" hurricane season. Whatever the reasons may be, it's likely that bookings in June were stronger than the rest of the summer.