Surely this depends entirely on the usage patterns of the affected systems? We have some servers that are used mainly in the morning (eg 7am - 2pm) so we can often do maintenance during the late afternoon. Other servers aren't used until later in the day so we can do stuff first thing in the morning. Still other servers are used mainly for overnight loading/processing so we can take those down in the middle of the working day.

I guess if you're supporting internal systems used by other office workers then you're mainly going to be doing maintenance overnight. But for systems used by clients who don't necessarily work 9-5 you need to be more flexible.