These have started to appear which is great, they are a big factor in the real world and are most welcome but we need a mechanism to help us supply the demand for those days only.
Having spare aircraft to meet Special Demand isn't going to work, but some of us do have spare (usually older) aircraft to cover 'C' and 'D' checks of the fleet and these could be assigned to a route (a newly created short term route or doubling up with another active aircraft?) for a limited period. What's needed from the over-worked Sami then is the option to define start and end dates for that aircraft on the temporary route.
This tweak would also allow my back-up aircraft to be assigned to cover the major maintenance of my regular planes by flying standard routes without having to move the route over to the back-up aircraft (usually too early because that's when you are on-line) (and then I often forget to move it back again).
A future development after Cargo is up and sorted perhaps but a very useful tool for the occasionally on-line players.