I would guess that is they work for some kind of public agency / health service they would be more likely to be on standard working hours with weekends off. But then some agencies have 24/7 emergency access available in the case of violent assault or something like that so I would suppose that there would be people rostered on over the weekends and in the middle of the night and the like.

If people are in private practice then they are able to choose, I would expect. It would actually make sense to me that someone in private practice would purposely try and work outside normal working hours so that people who do work can better access their services (e.g., working in the evenings and / or over the weekend). I think it really does depend on the therapist. You might well be able to find someone who works on Sunday if that is what you want. That being said I'm fairly sure that it would be a minority who would work over the weekend so it make it more likely that you could find someone if you lived in a big city somewhere.