It's up to the individual cleaner but would I go out just for window cleaning then probably not unless it was a very large job.
However I don't mind working sat or sun myself on domestics if that's the best time when customers will be home for power, access etc for a combined exterior cleaning services. Like for instance when they want say their guttering & downspouts checked & cleared, plastics, conservatory, deck, paving, patio and/or driveway cleaned. That's basically any larger jobs that are worth my while to do.
It also tends to be easier to pick up extra work whilst working weekends as more people are about and can see what I’m doing.
I've found being flexable is the key in this game as you can chose when you want to work or not and take time off during the weekdays too. That is the main reason why I don't have a large regular window cleaning round as I prefer not to to just clean windows .