I wouldn't be surprised if Jet2 kept some of the crew back ready to operate the call centre for the January/February booking rush, then when that dies down, redeploy them on the planes when the capacity ramps up for Mar/Apr and May/June
plane.silly - that’s plain silly.
There are some crews that are employed on contracts that sees them sees them laid off over the winter, but as a percentage it’s not that many. Some pilots take extra leave and go travelling/studying etc.