As I explained earlier in the thread, even though Sunday's are outside the working week, certainly on the GE side, rostered Sunday's are compulsory overtime. So even though they are outside the working week, it isn't optional to work Sunday's, it is mandatory.
The problem comes when you have someone who is off the road due ill health or because they have had an incident and thus can't work their rostered Sunday. Then you need to find someone to cover that job on a volunteer basis because we don't have spares on a Sunday generally. Or with the endless weekend engineering work additional Sunday jobs get created because diagram hours get eaten up having to taxi drivers from depots that are within a possession to work trains elsewhere on the network and you need volunteers to run the service.
What actually seems to happen is it's harder to cover the work on a weekday because Monday-Friday a far higher percentage of your drivers are already at work, the pool of people who are off who can work overtime is much smaller. And the number of jobs you need to cover can soon snow ball.
Let's say Driver Bloggs has had a minor incident, and today is the incident review meeting. Bloggs job needs covering, he has his union rep present to represent him, so union reps job needs covering too. H&S rep has a meeting with company management that day, so that drivers job needs covering too. You also have 2 drivers on annual leave days that day and one driver off sick following a fatality. You also have a DI training new shunters in the yard so the DI's job needs covering. Now you got 7 jobs you need to cover that day! You might have 3 spare drivers that day so you still got to find 4 people willing to work their day off. And that's why sometimes the 'xx:xx is cancelled because a member of traincrew is unavailable'.