Upfront customer requirements often say they want >99.5% uptime (which allows for 3.5h downtime a month anyway) or some such. In practice B2B customers often don't care much if hour-long downtimes happen every week during off-hours. Sometimes they're even ok when it gets taken down over a whole weekend. Things serving the general public have different requirements but even they have their activity dips during the late night where business impact of maintenance is much lower.