It isn't a broad question at all... I think it is rather to the point.
Generally, airlines have a somewhat limited ability to change equipment to match the potential loads. The more warning an airline would receive, the more likely that equipment could be changed to meet the need. For instance, every year US Airways might put additional wide-body capacity on CLT
to meet the needs presented by the big golf tournament at TPC Sawgrass. Obviously they would know about this way in advance. Now, if tomorrow John Kerry were to get busted with a mistress in Burlington, Vermont, then it is unlikely that the airlines would be able to change equipment that quickly to meet the demands that the hordes of reporters might place on that route. (Relax Kerry fans... Its just an example)
Allot goes into fleet planning. Airlines try to plan this stuff out as far in advance as possible because the maintenance schedules are rather strict. If the airline has some operational spares, there may be more leeway. However, in this day and age of cutbacks, most of us aren't operating with many spares.