The date/time for that deployment is approximately 14:30 Central time today, 2016-08-17.
In order to cut over from the old deployment to the new deployment of the Manager (and to do away with some long-standing hacks on the sysadmin side of the project), the application will be unavailable for approximately half an hour.
In terms of general traffic, Wednesday is the slowest day of the week for the Manager, so impact should be minimal.
The downtime warning will appear in the app shortly and it will go away when the maintenance/migration is completed.
Thanks again for using the manager!