When you want to ensure that every user who logs in to the system will see an announcement or inquiry, simply add your message to the /etc/motd file. When a user logs in, all messages in that file are displayed, as in the following example:
The system will be down from 1700 to 2300 hours on Friday, September 30, for upgrades and preventive maintenance.
Edit the /etc/motd file regularly to remove obsolete notices.