I have a lot of maintenance work to do today and this evening that involves updates, taking systems down for imaging, reboots to make a new libc and openssl active, and reboots of the main file servers to make various updates in them active.
Most of the heavy work that will impact customer access to mail, web, etc, will happen after midnight, first the update of the main file servers and then taking mail, ftp, and the more heavily used shell servers down for imaging. The less used servers that at the time have nobody logged as well as servers that are replicated will be serviced earlier.
It is suggested that you do not plan any uninterruptible activities between midnight and 1AM on 6/6/2015 as this is the time frame the main file servers will be rebooted and during that time all services will be unavailable for a period of approximately 20 minutes.