Nick's organization sets aside Saturday nights from 2 a.m. to 4 a.m. for scheduled maintenance. What is this type of reserved time typically called?

Preplanned times when maintenance is done and outages may occur is known as a maintenance window. An unscheduled outage is just that—unscheduled and unplanned, often due to a failure or other issue. Allocated downtime and allowed outages are not common terms for this.