Announce it early and more than once
Tell customers a week ahead, a day ahead and when it starts. Say the time in their time zone and what will not work.
Set a window and keep to it
State a start and an end. If you will overrun, post an update before the end time.
Prepare the replies
Write the answers for what is down, when it returns and what to do in the meantime, and load them as macros.
Staff the window
Have someone on support who knows the plan. Nothing frustrates customers more than an agent who has not heard of it.
Confirm and follow up
Post when service is back, check the first tickets afterwards and thank people who waited.