There's a default-off feature called "Agent Info" on the Agent side and "Customer Accept" on the Customer side.
The concept is that when a user logs in, they can't proceed until they acknowledge the notification. This can be used for things like informing customers that you're aware of a specific outage, or that all-hands event IT is having next Friday.
There are three parts: Enabling the module, establishing the "key" and providing the text interface.
The text interface is just updating Kernel/Output/HTML/Standard/CustomerAccept.tt (or .dtl if you're 3.3.x and below) or AgentInfo.tt (or .dtl), although these are default and customizable filenames. Do not forget that the acknowledgement link must be functional, though you can of course change the text within the <a></a> markup. Translation can be done through the normal Translation methods of OTRS. It is practically fully customizable as an HTML page, as well as using the template markup.
The Key field is a value that, when the user has acknowledged the current-key's message, is stored against the current user. This is to check if the user has already acknowledged the message. You may choose what you want for the key, but my suggestion may include a keyword reference and a date (network-7-7-15, for example). Note that this key needs to be unique for each message to be presented, and the message will be presented (and persistent) until the acknowledgement has been processed. The message will not show again until the key is changed, even if the message content changes.
The message is displayed next refresh of the web page, so this is a live/real time implementation. It doesn't involve restarting any services.
There is no "scheduled time to turn this feature on or off" option.
This video from Sean Beasley explains how to enable the three boxes in SysConfig for AgentInfo. The same concepts apply to CustomerAccept.