Responses
That's not a manga thing, it's a Japan thing. It's pretty common practice for companies in Japan to require salarymen to work 6 days a week, so that includes Saturdays. I read somewhere that there are still some companies them that give their employees the whole weekend. Other than that, Monday-Saturday is typically observed as a full work week in Japan.
In IT industry, sometimes you have to work saturday as well if we have deployments. But usually just 5x a week.
I never understood how office workers have to go to work on the weekends in manga. It's the weekend. Business days are Mon-Fri.