There is a regular patch schedule in effect with primary servers updated Mondays and Tuesdays as required around 03:00 (time-zone dependent). If a reboot is required there will be a short period of downtime of around 10 minutes, this will generally occur once a month on Monday. Secondary servers are updated Wednesdays and Thursdays as required around 03:00 with no downtime.
Other maintenance tasks which require downtime will be notified in advance, timing of notice varies depending on the urgency and risk involved but will generally be at least a week. Major maintenance is normally scheduled outside busy academic periods.