A notion in system administration
Scheduled downtime is a period in which a system is down as planned.
While the administrator should strive for 100% uptime
at least during hours that the system is likely to be used, scheduled downtime,
with reasonable advance notice to the users, is infinitely preferrable to having a system go down spontaneously or abruptly, due to fatal errors, or interventions that cannot possibly be postponed any longer.
To illustrate the advantages of scheduled downtime, consider the question whether a regular reboot of a system is a good idea. Many system administrators will meet this question with a knee-jerk response: if the system is doing fine, why destroy its uptime by rebooting it for no good reason?
But if you work with many users whom you may not be able to contact personally on all occasions, it's practical to have a regular scheduled reboot - this gives you the guarantee of having a fixed service window at the moment the planned downtime occurs, leaving little room for users to be unpleasantly surprised and bother you at a time when you would really like to install those new system patches or cleaned up boot scripts instead.
Scheduled downtime buys you opportunity at the expense of uptime.