That's the biggest pitfall of your proposal. Those are the times these "casual" staff will likely want off too. Back to the drawing board I'm afraid. The most sensible solution would remain bringing all days inside the working week, but you will never resolve the issue of a higher than normal absence level at certain times of the year. To cater for that you need a bigger establishment of crew, which isn't going to happen until the question of who funds it is settled. That goes way beyond the level the railway industry alone can resolve.