There are certain times when you are much more likely to see reservations going back into inventory as people cancel them.
First, around the 60-70 day mark--when people make their fastpass reservations and have to move around their park days depending on what fastpasses they were able to get.
Second, around 30 day mark where resort package reservation final payments are due.
Third, a week before the trip,, when people cancel their room-only reservations before the 5-day deadline, or when they start to second guess their plans as it becomes more real (i.e., whoa, maybe we have wayy too many character meals and it is going to blow our budget).
Finally, the night before, when people cancel their dining reservations to avoid the midnight deadline before incurring $10 per person charges when they change their plans for the following day (maybe because they weren't able to get a ROTR boarding group and need to go back to Hollywood Studios to try again, or maybe someone in their party gets sick and they don't want a big expensive meal).