Many years ago I was told by somebody in Britannia planning that you sell the seats first and then worry about what will do the flying later. There were three sources - your basic BY fleet, third party a/c leased in for the season (back in the day the likes of Air Florida and Quebecair, nowadays Sunwings and the east eiuropeans) and third part a/c doing specific flights on a season long basis (as described by WHBM at #507).
Certainly in the regions, the fleets doing short haul sun flights were relatively stable with the same a/c operating from the same airport for weeks/months at a time until they needed to be rotated for maintenance/fleet balancing purposes as described by Smudge and LBAflyer in previous posts.
Of course that's the plan - but as we know, things quite often "turn to sh*t" (a technical phrase...) and you start robbing Peter to pay Paul to keep things moving and a/c will be switched around to try and keep the schedule on track - the last resort will be subbing in at short notice third party carriers to operate specific flights.
This year seems to have got off to particularly difficult start, with far more switching around of the basic TUI fleet between UK bases and subbing in of third parties to cover specific flights. Seems to be several possible causes - all or some of: season long leases falling through, delays affecting getting the BY fleet back to capacity (e.g. the returning SWG leases) and staffing issues. Hopefully things will settle down as the season develops.