Maintenance windows for updates that may create momentary downtime are weekly 9pm-midnight PST on Thursdays by default and you would receive a two days notice. The frequency of these is fairly rare (about 3 times per year).
If the Thursday evening time slot doesn’t work for you, we have four alternative windows: 7pm-9pm, 12am-3am, 5am-7am, 9am-11am. You would specify your preference for one of the alternative slots via support ticket.
Note: This schedule does not apply to any of the High Availability (HA) plans, as the dual-node nature of the HA setup allows us to update one server while the other stays active — i.e. there is no downtime. That said, the WP Admin control panel would be inaccessible for roughly five minutes while the upgrade occurs on the primary web node.