I do work most Saturdays, it can get very tiring but I would lose my mindee otherwise in the week too. I do occasionally do a 7 day week but rarely and really wouldn't recommend it.
I don't find it too bad working a Saturday as I make sure the parent realises that it's just a go with the flow day, nothing major planned, just pottering about as one does on a Saturday. She is more than happy with that.
If I had little ones of my own about the place I wouldn't do it though.