In terms of days off being fixed, they are inasmuch as once the roster is written, the days off won't be changed. You get three Request Days Off per month which are pretty much guaranteed to remain unchanged. Rosters are published on the first Thursday of each month for the next month.
In my time at Jet2, the program has gotten progressively busier. Last year I flew ~240 sectors. This year I've already done over 220 with 5 months left to fly.