SQL Server Maintenance Windows - mini DBA
The maintenance windows tab of the alerts screen has a section for daily and weekly maintenance windows:
This windows will be active for the amount of hours, at the time you specify on every day of the week. It is often used when nightly database backups are run leading to decreased performance.
This window will only be active on the day specified. It is often used over weekends when patching and/or index rebuilds take place that will trigger alerts.
Setting a duration of 0 hours (the default) will cause the maintenance window to not ever be active.